This clobbers the functionality from commit:

    06d73142

    make idle timers actually activate only when idle

Since now all rendering and re-sizing is done in idle handlers it
does effectively the opposite of what was intended. A better solution
would allow special-casing the processing of just rendering,
re-sizing and window management to spin for eg. progress bar
rendering.

Also add helpful debugging labels to the idle & timeouts.

Also cleanup the Idle vs. Scheduler handling.

Also ensure that starting an Idle triggers a mainloop wakeup.

Also add a unit test.

Change-Id: Ifb0756714378fdb790be599b93c7a3ac1f9209e6