deselect after use if nothing was selected before

in AddRemoveBreaks and ResequenceRehersalMarks.
Also prevent
ResequenceRehersalMarks to destroy a list selection,
AddRemoveBreaks
prevents that already, neihter works or makes sens on a list
selection.
Furtermore prevent a crash when running AddRemoveBreaks or
SlashFill on an empty score
(for some reason ResequenceRehersalMarks and
SlashRhythm don't have that issue).