GTK: Implement global shortcuts

[pluiedev]

Should work on every platform that implements the XDG Desktop Portal (see https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.GlobalShortcuts.html)

It's a lot of D-Bus sadness, but it should be doable.

[xordspar0]

Gnome 48, which shipped this week, now supports the global shortcuts portal.

[digitalsignalperson]

My previous notes saved about the dbus interface:

For kde it's under the org.freedesktop.impl.portal.desktop.kde
For gnome it sounds like maybe org.gnome.GlobalShortcutsProvider

Poking at it with D-Spy under the org.freedesktop.impl.portal.desktop.kde bus, /org/freedesktop/portal/desktop path, org.freedesktop.impl.portal.GlobalShortcuts. Also trying to register something with dbus-send or python dbus, but I give up for now

[pluiedev]

To my knowledge org.freedesktop.impl.portal.desktop.kde and org.gnome.GlobalShortcutsProvider are private interfaces that we shouldn't interact with. The public API has always been just org.freedesktop.portal.GlobalShortcuts.