MicroPython: Add start() and quit() functions.

[Gadgetoid]

New API changes

• start() - enters a blocking main loop, calls MICROPY_EVENT_POLL_HOOK to keep subsystems running
• quit() - signals start() to break
• Add start() to examples in place of while True: tick()
• Fix launcher crashing when buttons pressed during startup
• Add "quit" menu item to launcher, to unblock for Thonny
• Reset "tick" on "quit" so applications launched from the launcher don't start at weird tick offsets

start() enters a blocking main loop that dispatches update/draw calls to the Python functions.

When quit() is called, start() will drop out of the loop, clean up the cached update/draw pointers and reset "tick".

Your scripts should now look like this:

def update(tick):
    pass

def draw(tick):
    pass

start()

tick() and _reset() are included for posterity, but these may both be deprecated since _reset() happens implicitly when quit() is called.

Stats & GC

• Fix calculation of "tick" us stats
• Above also fixes FPS counter
• Fix tick being double-incremented, and fix launcher to account for this
• Run the C equivalent of gc.collect() in the main loop
• Try and sleep the is_flipping() loop a little... my PicoSystem is getting mighty warm!

:thinking: Flipping, at least, should be a pretty constant time operation, can we stick a larger sleep value in here?

OC/Buffer

• Overclock early to avoid an on-init change of clock doing bad things
• Give the SCREEN buffer back to C exclusively, MPY reduces the gc_heap to accommodate this change
• see: https://github.com/pimoroni/micropython/commit/f826b9e7a1921604b4cd3e1911cdaed3a380496b