Provide window ref access from GLFW shell's plugin registrar

[stuartmorgan-g]

This would allow plugins to call methods that require a window ref. Currently there aren't many, and several use cases should be solved with system channels instead (window title, window size/positioning), but it's plausible that more will be added over time.

This will require a bit of header restructuring; the registrar is separate from the details of the windowing implementation so that it can be re-used, but we'll need a solution to allow shell-specific things.

[stuartmorgan-g]

This was fixed by flutter/engine#8806; I forgot to tag this issue in the description.

https://github.com/google/flutter-desktop-embedding/blob/master/plugins/window_size/linux/src/window_size_plugin.cc is an example of using it.

[github-actions]

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