x11: Implement inhibit_system_shortcuts API

x11: Implement inhibit_system_shortcuts API

On X11, there is no such equivalent to the inhibit shortcut protocol
found on Wayland.

To implement the inhibit_system_shortcuts API on X11, we emulate the
same behavior using grabs on the keyboard.

To avoid keeping active grabs on the keyboard that would affect other
X11 applications even when the surface isn't focused, the X11
implementation takes care of releasing the grabs as soon as the toplevel
loses focus.