Measure flag for system-initial double bar line

Adds to the Measure class a flag which turns the system-initial bar line to double when the measure is the first of a system.

This is needed by some engraving styles (mostly jazz? @MarcSabatella raised the point); adding this flag allows:
- to set the system-initial bar line to double independently of actual system breaks (the flag triggers only if the measure is the first of a system and does nothing otherwise)
- to make the system-initial bar line non-selectable (and then non-editable), simplifying the bar line management.

UI for setting the flag is added to the Measure Properties dialogue box; advice on text used is welcome.