Changed removal of spanner in range selections.
Slurs are removed when start or end segment are removed.
Other spanners are removed if the start segment is removed.
Added test suite for selection range delete.
Added tests for range delete of spanners, and slurs.
Changed highlighting for spanners.
Merge pull request #993 from BartlomiejLewandowski/range-selection-spanner-delete

Range selection spanner delete