Currently, the Linux embedder does not handle window exposure events.
This is typically not a problem for users who use compositing window
managers, since they keep the display buffers even if the window is
completely covered. However, for users that don't use a compositor, the
window will not be redrawn by the engine if it was previously covered
until another event triggers the redraw.

This patch implements the GtkWidget draw callback to handle window
exposure events. The callback doesn't actually draw anything, it just
schedule a frame for drawing by the engine.

The engine doesn't support exposure events, so instead, we force redraw
by sending a window metrics event of the same geometry. Since the
geometry didn't change, only a frame will be scheduled.

Switches the Windows embedding from the standard C API to the new proctable version, to allow for unit testing of the embedding layer separately from the embedder APIs implementation. This includes moving some engine messaging that was still in flutter_windows to the C++ engine class to better encapsulate the proc table.

and roll Dart to 52783837369de45d3372cb6c6b7cdd63e71cd829.

Converts the macOS embedding to use the new proc table version of the embedding API, and adds one example unit test using it to demonstrate and validate the approach.

Bug:
  https://github.com/flutter/flutter/issues/67579

The framework handles arrow keys, delete, and backspace (and with better
unicode support), so we shouldn't handle them at the embedding level.

Fixes #69202