Apps hang temporarily on resume from sleep

[Plywood9]

If a Textual app is suspended for some reason - typically because the device it's running on goes to sleep - then it will temporarily hang on resume from sleep and refuse to update or accept input. This is typically a minor stutter after a brief sleep; after being left overnight it can be several seconds; left for long enough it becomes minutes.

This occurs because of the loop here:

textual/src/textual/timer.py

Lines 152 to 157 in f5be1b7

	while _repeat is None or count <= _repeat:
	next_timer = start + ((count + 1) * _interval)
	now = _time.get_time()
	if self._skip and next_timer < now:
	count += 1
	continue

If a timer with skip enabled has fallen behind, then it busy loops until it catches up. This is fine if it's only a little behind, but rapidly becomes expensive if it's a lot behind. This busy looping behaviour happens in, for example, the "screen_update" timer which fires every frame. An app which has been suspended racks up about five million ticks a day, enough to produce noticeable lag while the busy loop catches up.

This is easy enough to fix - just calculate the next required timer fire rather than busy looping.

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