$ cat mouse_modes.txt
Steps
=====
[X] Add mouse mode base class (MouseModeSelector).
[X] Add such a mode to the window as 'current selector'.
[X] _window_manager tracks top-most selector. (Active selector.)
[X] Selector can have ground cursors at some position.
[ ] The 'active' selector gets inserted into rendering.
[ ] Window of the Selector may track mouse position, and update rendering position.
[ ] Window of the selector gets clicks.
[ ] Window of the selector gets wheelies.
[ ] Terraform GUI
[ ] Selector adds voxel contents to the rendering with its own recolouring
[ ] Fences GUI
[ ] Path GUI
[ ] Coaster GUI