Add 'Add-ons' help section, dealing with add-on types and usage of the manager

Remaining items to do with this:

  * Add a quick explanation about uninstalling add-ons

  * Review for style/content/helpfulness

  * Implement a Help button in the manager (this somehow breaks
     spectacularly atm, hence not committed)

data/core/help.cfg

@@ -1,7 +1,7 @@
 #textdomain wesnoth-help
 [help]
     [toplevel]
-        sections=introduction,gameplay,units,abilities_section,traits_section,weapon_specials,factions_section,terrains,commands,encyclopedia
+        sections=introduction,gameplay,units,abilities_section,traits_section,weapon_specials,factions_section,terrains,addons,commands,encyclopedia
         topics=about,license
     [/toplevel]
 
@@ -65,6 +65,12 @@
         sort_topics=yes
     [/section]
 
+    [section]
+        id=addons
+        title= _ "Add-ons"
+        topics=..addons,using_addons,managing_addons
+    [/section]
+
     [section]
         id=commands
         title= _ "Commands"
@@ -778,6 +784,57 @@ For those who go by land or sea, a bridge is the best of both worlds — for gam
     [/topic]
     #wmllint: markcheck on
 
+    #wmllint: markcheck off
+    [topic]
+        id=..addons
+        title= _ "Add-ons"
+        text = _ "Wesnoth’s extensibility has allowed the user community to develop much content over time making use of the full potential of the game engine to deliver new scenarios and campaigns beyond what the official content set has to offer.
+
+"
+        generator="contents:addons"
+    [/topic]
+    #wmllint: markcheck on
+
+    #wmllint: markcheck off
+    [topic]
+        id=using_addons
+        title= _ "Using Add-ons"
+        text= _ "The game supports various types of add-ons that are used differently according to their purpose." + _ "
+
+<header>text='Campaigns and Scenarios'</header>" + _ "
+
+Single-player campaigns are collections of scenarios that fit together to tell a story. Both stand-alone scenarios—if intended to be played as such—and regular campaigns are available from the <italic>text='Campaigns'</italic> menu in the title screen." + _ "
+
+<header>text='Multiplayer Campaigns, Scenarios and Map Packs'</header>" + _ "
+
+<italic>text='Multiplayer'</italic> games can be played on fully customized, scripted scenarios, or even specially designed campaigns. There are also packs providing sets of individual multiplayer scenarios not connected in the form of a campaign." + _ "
+
+<header>text='Multiplayer Eras and Factions'</header>" + _ "
+
+For gameplay purposes, the various races of creatures in the world cooperate with each other in factions. These factions are grouped in balanced sets with a common theme; for example, the main factions of Wesnoth can be found in the included Default Era.
+
+In <italic>text='Multiplayer'</italic> mode, you can choose an era when creating a new game, and players can pick from the available factions for that era when setting up their sides and teams." + _ "
+
+<header>text='Creator Resources'</header>" + _"
+
+Content authors can use resource packs available in the add-ons server to enrich their own content with existing assets such as images, music and code. These are not generally intended for the players’ use and cannot be directly used in the game on their own; however, other playable add-ons may depend on them and suggest or require their installation during download."
+    [/topic]
+    #wmllint: markcheck on
+
+    #wmllint: markcheck off
+    [topic]
+        id=managing_addons
+        title= _ "Installing Add-ons"
+        text = _ "User-made add-ons can be obtained and updated through the <italic>text='Add-ons'</italic> option in the main menu. After connecting to the add-ons server (by default <italic>text='add-ons.wesnoth.org'</italic>), you will be presented with a list of add-ons available in the server for downloading.
+
+To search add-ons by keywords, just type them in any order in the <bold>text='Filter'</bold> box, separated by spaces.
+
+To install an add-on, select it from the list and click on <bold>text='OK'</bold>, or simply double click on the add-on’s title. The <bold>text='Description'</bold> button provides you with additional details about the add-on, such as its full description, installation status, and available languages.
+
+By default, you will see the <bold>text='All Add-ons'</bold> view, displaying all add-ons regardless of whether you have them installed or not. You can click on the button to change the view as necessary. The <bold>text='Upgradable'</bold> add-ons view provides you with the option to upgrade all your installed add-ons whenever applicable. Use the <bold>text='Not Installed'</bold> view to display only those add-ons you have not yet acquired from the server."
+    [/topic]
+    #wmllint: markcheck on
+
     #wmllint: markcheck off
     [topic]
         id=..commands