data: Add schema for global shortcuts

Adopt this piece of data so far, even though gnome-settings-daemon is not directly involved with it. At least until we move handling of shortcuts altogether to gsettings-desktop-schemas.

Since the format for shortcut IDs is undefined and might not be able to be stored as DConf path components, avoid the temptation to unroll the configuration, and store settings as a "a(sa{sv})" containing the configuration for all shortcuts. This GVariant format coincides with the response format that xdp-gnome expects from the UI.