Gathering points in FloorMapEditor & more

Added material gathering points to FloorMapEditor map display, for now
right-clicking opens the TableEntryEditor and selects the gathering
point's node for editing, needed slightly messy workaround for when
TableEntryEditor is not yet initialized; added gathering point overlay
toggle to FloorMapEditor, generally rearranged and cleaned up the
control a bit; bugfixes and improvements to TilePalettePair,
FontRenderer and others