Implement disable_mode property for TileMapLayer physics

[soap-cheese]

Currently the CollisionObject2D class has the property disable_mode, which determines its behavior in physics when process_mode is set to ...DISABLED (or to ...INHERIT and the parent node is disabled). By default, the object is removed from the physics simulation.

TileMapLayer can be a part of the physics simulation as well, via physics layers of its TileSet. However, since TileMapLayer doesn't inherit from CollisionObject2D, it does not have disable_mode property, or (as far as I'm aware) any reasonable alternatives to it. Default behavior is to not remove the TileMapLayer node from the physics simulation when its processing is disabled. It causes collisions regardless of whether it can process or not.
This is inconsistent with existing logic, and also makes it impossible to effectively disable large scenes at once by disabling processing of the root node, which seems like the whole point of Node.PROCESS_MODE_INHERIT option in the first place.

I suggest it would make sense to add disable_mode property either to TileMapLayer, TileSet or individual physics layer of a TileSet, with the same functionality as in the CollisionObject2D class. It would be set to "remove from simulation when disabled" option by default, and could be set to "make static" or "keep active", the same way it works with CollisionObject2D.