RFC: add IsValueChangedFromSaved to VCL button and listbox

Add a method
   bool IsValueChangedFromSaved()
to the various Button and Listbox classes.
Use it to simplify code like
   if ( maBoldAppFont.GetSavedValue() !=
        TriState(maBoldAppFont.IsChecked()) )
to
   if ( maBoldAppFont.IsValueChangedFromSaved() )

Change-Id: I26b291d0980237497063ee301d63a49c7d2bc227
Reviewed-on: https://gerrit.libreoffice.org/9250
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>