wm/monitors: First step of extrating monitors from wm.

Renamed all instances of window_manager to monitors.
wm/monitors: Renamed files.
w/monitors: Updated new filenames in code.
monitors: s/add_monitor/add.
monitors: s/remove_monitor/remove.
monitors: s/monitor_from_rect/from_rect.
monitors: Simplified and cleaned up api.

Finall fixes #1.