shortcut-label: make it public

shortcut-label: make it public

GtkShortcutLabel is a widget that displays a single
shortcut accelerator or gesture in the user interface,
and is currently used by the shortcuts window.

This widget, however, has public value as other applications
also may want to expose their own shortcuts. For instance,
it'll be useful for the Keyboard panel on Control Center and
the new shortcut editor in Pitivi, among others.

This patch exposes GtkShortcutLabel as a public widget,
and adds the necessary documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=769205