Implement GlobalShortcuts portal #185

Problem

Applications like Chrome¹ and Electron apps² (and others) use the GlobalShortcuts desktop portal to implement global shortcuts under Wayland, allowing apps to provide keyboard shortcuts when they do not have focus. Elementary OS does not support this.

Proposal

Implement https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.impl.portal.GlobalShortcuts.html

Prior Art (Optional)

• GNOME: https://gitlab.gnome.org/GNOME/xdg-desktop-portal-gnome/-/merge_requests/208
• KDE: https://invent.kde.org/plasma/xdg-desktop-portal-kde/-/merge_requests/80