Made robot 'modes'

This is a hacky/incomplete set of changes, because
   a lot of the stuff that's going to be made modal -
   specifically building things - still isn't modal.
That's because I'm going to do a separate refactoring
   of the building process.
Standardized robot modes, made building modes.

Code changes:
Instead of "get_building_tile" we have
   "get_current_click_action", which is a pretty
   hacky seeming concept, but works with our current
   UI rather well. There's a lot of hackiness sitting
   there (and in the modes) so it could use some
   cleanup at some point.
Deconstructing buildings isn't implemented yet,
   but it will be easy to do so.

In-game changes:
All actions are now the mouse button.
   (Well, except moving and mode switching.)
Rotating conveyors is now "click one while in conveyor
   building mode".
The target marker is better and color-coded.
Lasers have a (inaccurate I guess) target marker.