add is_buildable_on_exits option

[sanslash332]

The previous feature of allow a building be builded on a exit to block the pass of enemies was very interesting, or give to you a greats options to modding, for create interesting things like buildings that can be or a regular building, or a gate, for example.

In the last version, we only have is_buildable_on_exits_only property, that is awesome and works fine for walls and gates, but for example add an is_buildable_on_exits for a small buildings like farms maybe a good option :)

With that, you increase the options. buildings only for meadows, buildings for any subsquare (towers) buildings onli for exits or any convination of that. exits and anywhere, etc.

:)