Make undo work again, undeprecate on_event()

Fixes #6898. The issue is that non-WML events added through the new events API
always disable undo with no equivalent of WML's `[allow_undo]`. The long-term
fix is to add a way to do that; however until that's available then listeners
for `moveto` need to use the old `on_event` API. The old `on_event` API can't
be deprecated yet, and this is enforced by our unit tests (the build fails if
there are unexpected deprecation warnings during the tests).

Reverts most of 7e234f8833. Does not revert files
that only listen for non-undoable events such as `die` or `new turn`.

Reverts the deprecation part of #5663's 8cd1332630.

changelog_entries/fix_undo.md

@@ -0,0 +1,2 @@
+ ### Miscellaneous and bug fixes
+   * Fixed not being able to undo moves (bug #6898)

data/lua/diversion.lua

@@ -1,6 +1,7 @@
 
 local _ = wesnoth.textdomain 'wesnoth-help'
 local T = wml.tag
+local on_event = wesnoth.require("on_event")
 
 local u_pos_filter = function(u_id)
 
@@ -99,7 +100,7 @@ function wesnoth.wml_actions.on_undo_diversion(cfg)
 	status_anim_update(true)
 end
 
-wesnoth.game_events.add_repeating("moveto, die, recruit, recall", function()
+on_event("moveto, die, recruit, recall", function()
         status_anim_update()
 
 end)

data/lua/on_event.lua

@@ -51,5 +51,5 @@ local function on_event(eventname, priority, fcn)
 	end
 end
 
-core_on_event = wesnoth.deprecate_api("on_event", "wesnoth.game_events.add_repeating", 1, nil, on_event)
-return core_on_event
+core_on_event = on_event
+return on_event

data/modifications/pick_advance/main.lua

@@ -1,6 +1,6 @@
 -- << pick_advance/main.lua
 
-local on_event = wesnoth.game_events.add_repeating
+local on_event = wesnoth.require "on_event"
 local F = wesnoth.require "functional"
 local T = wml.tag
 local _ = wesnoth.textdomain "wesnoth"

data/multiplayer/scenarios/2p_Dark_Forecast.lua

@@ -1,7 +1,7 @@
 
 local _ = wesnoth.textdomain 'wesnoth-multiplayer'
 local T = wml.tag
-local on_event = wesnoth.game_events.add_repeating
+local on_event = wesnoth.require("on_event")
 
 local random_spawns = {
 	{

data/multiplayer/scenarios/2p_Isle_of_Mists.lua

@@ -1,6 +1,6 @@
 local _ = wesnoth.textdomain 'wesnoth-multiplayer'
 local T = wml.tag
-local on_event = wesnoth.game_events.add_repeating
+local on_event = wesnoth.require("on_event")
 
 local random_spawns = {
 	{