add skeleton for the wall placement mechanics
While not sounding particularly spectacular, quite a bit happened under the bonnet here.
The concept of specific BuildModes was introduced when inside the Build game mode (remember game modes?), so we always know how to handle input.
This also adds code to make the markers clickable. Furthermore, once in wall placement mode it will also draw the walls in the “lower” setting, so that you can actually see what you are doing. It will also draw the marks in a way to completely ignore drawing depth and just draw them on top of everything else, once again, so you can always see what is happening.
These SelectionIndicators, as they are called internally, also get a stub function so that we know where to draw them already, as you can tell, it does not do much yet.
