Fixes#1052
This implements a `close_all_windows` binding in the core and implements
it for macOS specifically. This will ask for close confirmation if any
surface in any of the windows requires confirmation.
This is bound by default to option+shift+command+w to match Safari. The
binding is generall option+command+w but users may expect this to also
mean "Close All Other Tabs" which is the changed behavior if any tabs
are present in a standard macOS application. So I chose to follow Safari
instead.
This doesn't implement this feature for GTK, that's left as an exercise
for a contributor.
Fixes#1052
This implements the about window as a custom window with a view
controller. This lets us implement the proper responder chain so that
our custom close window IBActions do the right thing.
This has an additional benefit that we can easily customize this window
going forward.