It seems the controls aren't linked with the labels in the outline numbering
dialog box (Tools - Outline Numbering). Add info about mnemonic_widget to the
labels.

There is still a pending issue with separator before/after which I guess
should be grouped in a frame, but I'm not confident enough with Glade or
it's ui format to try and fix it at the moment.

Change-Id: I8dffef703ba03dca31a356426aa5b8a9bc1103be
Reviewed-on: https://gerrit.libreoffice.org/14255Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>