Fix some load/save issues with Triggers

[zzam]

Fix some load/save issues with triggers Fixes:

• Load will create the original number of triggers (not doubled)
• Load: No bad memory access checking for removed triggers
• Load: removed triggers are now really removed
• Save: The removed-status is correctly saved even if it was from loading a file

[zzam]

I will do further changes to triggers (in a separate PR). Is it possible to use the LuaCallback from https://github.com/Wargus/stratagus/pull/624 also for triggers?