flap: Redo the Esc shortcut and make it managed

Merged Alice Mikhaylenko requested to merge wip/exalm/flap into main

GTK4 provides nice new shortcut machinery, let's use it. This allows us to make the shortcut managed instead of local and finally be able to close the flap with Esc when it's not focused, e.g. from the flap toggle button in the demo.

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