UI mouse logic

- Built some framework system to check mouse coordinates and active UI
panels
- If a UI is drawn on top of the map, the coordinates on the map (and
finding object) aren't shown
- This will allow reading the mouse relative to the UI panel it's on

Future:
- the idea was to add monsters' facing, but to add that I needed a way
to display it. I decided to go with some overlays, but to not show
overlays on top of UI panels, or still triggering below, needed to make
the mouse relative to the surface it's on.