Addon Manager: rename MP Modification type to just Modification

Mods can now work in either SP, MP, or both depending on how they're configured,
so this label is now misleading.

Do note this doesn't touch the _server.pbl type= key, which is still "mod_mp".

Closes #1475.

src/addon/info.cpp

@@ -165,8 +165,8 @@ std::string addon_info::display_type() const
 		return _("addon_type^MP scenario");
 	case ADDON_MP_CAMPAIGN:
 		return _("addon_type^MP campaign");
-	case ADDON_MP_MOD:
-		return _("addon_type^MP modification");
+	case ADDON_MOD:
+		return _("addon_type^Modification");
 	case ADDON_CORE:
 		return _("addon_type^Core");
 	case ADDON_MEDIA:

src/addon/validation.hpp

@@ -49,7 +49,7 @@ enum ADDON_TYPE {
 	ADDON_MP_ERA,		/**< Multiplayer era. */
 	ADDON_MP_FACTION,	/**< Multiplayer faction. */
 	// NOTE: following two still require proper engine support
-	ADDON_MP_MOD,		/**< Modification of the game for MP. */
+	ADDON_MOD,		/**< Modification of the game. */
 	//ADDON_GUI,			// GUI add-ons/themes.
 	ADDON_MEDIA,		/**< Miscellaneous content/media (unit packs, terrain packs, music packs, etc.). */
 	ADDON_OTHER,		/**< an add-on that fits in no other category */

src/gui/dialogs/addon/manager.cpp

@@ -227,7 +227,7 @@ const std::vector<std::pair<ADDON_TYPE, std::string>> addon_manager::type_filter
 	{ADDON_MP_MAPS,        N_("addons_of_type^MP map-packs")},
 	{ADDON_MP_ERA,         N_("addons_of_type^MP eras")},
 	{ADDON_MP_FACTION,     N_("addons_of_type^MP factions")},
-	{ADDON_MP_MOD,         N_("addons_of_type^MP modifications")},
+	{ADDON_MOD,            N_("addons_of_type^Modifications")},
 	{ADDON_CORE,           N_("addons_of_type^Cores")},
 	{ADDON_MEDIA,          N_("addons_of_type^Resources")},
 	// FIXME: (also in WML) should this and Unknown be a single option in the UI?