This adds a widget called `CcKeyboardShortcutRow`, sub-classing
`GtkListBoxRow`, to handle a shortcut row. This makes the implementation
a bit tidier, rather than handling it all in `CcKeyboardPanel`, and
allows the widgets that compose the row to be laid out in xml.
This is a prerequisite for moving the shortcuts to a new dialog.
The current CcKeyboardPanel used to manage keyboard
editing through GtkCellRendererAccel, which was replaced
when we moved to use a GtkListBox. Because of that,
the ability to edit shortcuts is now missing.
Re-add shortcut editing capabilities through a new dialog,
which is done according to the latest mockups available.
https://bugzilla.gnome.org/show_bug.cgi?id=769063