X11: Add support for window closing and quitting.

[xStrom]

This PR adds support for closing individual windows. Previously closing any window would close all of them. This can be tested with the multiwin example.

Additionally I refactored some code and added more inline documentation.

[xStrom]

I also implemented Application::quit. Needed to create an events-only hidden window for this, because there doesn't seem to be a way to receive events when all windows have been destroyed. With macOS and Windows the implementation allows for the druid-shell user (e.g. druid) to choose the policy of whether to quit when all windows have closed or keep running.

I also added a Quit app button to multiwin to allow testing of this functionality on X11, because there is no menu support yet.

[luleyleo]

Thanks a lot for this! Makes the X11 shell pretty much even with GTK, except for menus of course.

[xStrom]

I added more code comments to hopefully make things clearer. I also removed connection flushing from request_redraw and handled the cairo context creation error.

[luleyleo]

Looks good! The doc-links are definitely useful.