$  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