maps: Added ability to add coordinate based flags to maps

maps: Added ability to add coordinate based flags to maps

Reworked how tile-based flags are read.
They are now read from a property called flags with a simple int value.

Coordinate based flags are read from layer properties.
A layer property with int value called flags_x,y will be or'd with the
tile property value to arrive at a final flag value.

This can be useful for exits to allow the setting of exit table offsets
in the upper nibble of the flags byte.