just the straight-forward MessageDialog cases first

a) remove border_width from message dialog .ui so as to take
the default border width
b) retain 12 as default message dialog border for vcl widget case
c) remove layour_style from message dialog button boxes so as to
take the default mode (a no-op for vcl widget case)
d) use gtk response ids (vcl builder will converts to vcl ones)

Change-Id: I7de281093a1b64f92f71ca11e7cbba42bb658154
Reviewed-on: https://gerrit.libreoffice.org/50143Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>