keyboard: introduce CcKeyboardShortcutEditor

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