deselect after use, if nothing was selected before

in AddRemoveBreaks and ResequenceRehersalMarks.

Also prevent ResequenceRehersalMarks to destroy a list selection,
AddRemoveBreaks prevents that already, neither works or makes sense on a list
selection.
Furthermore prevent a crash when running AddRemoveBreaks or SlashFill on an empty score
(for some reason ResequenceRehersalMarks and SlashRhythm don't have that issue).