Fix #20782 - Adding or removing a section break (with a "Restart from meas. 1" default) does not immediately renumber measures.

Added calls to Score::renumberMeasures to relevant cases of MeasureBase::add() and MeasureBase::remove(). This also ensures that measures are renumbered when the property of the section break is changed.
Merge pull request #331 from mgavioli/Fix_20782_Section_break_does_not_renumber_measures

Fix #20782 - Section break and measure renumbering