keyboard: add API to track whether a shortcut is default

The current keyboard item API does not track whether the
keyboard shortcut is the default value or not. In order to
properly implement the Reset operation, the keyboard item
must receive this API and ideally handle it internally.

This patch adds the necessary API to CcKeyboardItem to track
whether the shortcut is the default value or not.

https://bugzilla.gnome.org/show_bug.cgi?id=769063