keyboard: add support to reset shortcuts to their default values

Following the proposed mockups, the shortcut list must
have the ability to reset modified to non-default shortcuts
right from the listbox.

After adding the necessary API in CcKeyboardItem, adding
the user-visible elements to enable that is easy.

To make that happen, add a button that resets the
keyboard shortcut.

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