keyboard: fix editing of custom shortcuts

After introducing the reset button to match the mockups [1], the
shortcut editor dialog had some issues exposed. This is visible
e.g. when the user tries to edit a custom shortcut's name and
the shortcut is disabled.

This happens because we assume there is always a shortcut set.
When we open the dialog to edit a custom shortcut, however, nothing
is actually set, and we end up saving the disabled shortcut when
editing the shortcut's name or command.

Fix that by initializing the shortcut's accelerators when editing a
shortcut, and correcting the logic to validate the shortcut.

[1] https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/system-settings/keyboard/keyboard-wires.png

https://bugzilla.gnome.org/show_bug.cgi?id=777845