Want to include `SubApp` in a system function,
but `ExclusiveFunctionSystem` requires `Sync`.

Moved the responsibility for running top-level schedules
from `App` struct to the runner function.

Set `ControlFlow::WaitUntil` to the next tick update or redraw,
whichever is closer.

Prepare for changes in
rust-windowing/winit#2900.

Send events for control from resource `WinitHandler`,
which contains `EventLoopProxy`.

The top schedule was redefined so that fine control would not be impossible.

- Schedule `Main` was split into `StartupFlow` and `UpdateFlow`.
  - The names of the sub-schedules (`Startup`, `Update`, etc.) have not been changed, so in many cases no migration is necessary.
- Add `Control` schedule.
  - Each time an event is received from windows and devices, this schedule is run.
  - This is useful for responding to events regardless of whether tick updates take place.
- Add `FrameReady` schedule.
  - Each time a frame is ready to be updated, this schedule is run.
  - This is the best place to decide whether to redraw.
- Rename `Render` schedule to `RenderFlow`.

Schedules that run sub-schedules are now named uniformly as `***Flow`.