Add warning for triggers using Building Exists incorrectly

[JoyfulShush]

Adds a new warning when saving maps that should help mappers in case they use the Building Exists event incorrectly. Only applies to SP maps.

When saving the map, if the map is likely to be a SP map, checks all triggers and sees if they have the Building Exists event. Any trigger that have this event is checked for the following:

• Gets the trigger's house. If the house is under the player control, skips checking the trigger.
• Checks what is requested in the Building Exists. Then makes sure the AI should have it at some point via one of the following checks:
  • Building for the house is pre-placed in the map.
  • House has the building as a Base Node. (note: we don't check that that those base nodes will actually be activated or can be built - too complex)
  • House has a unit that deploys into that building (e.g. MCV -> ConYard) in one of its TeamTypes.

Added English translations keys.