Style of the initial bar line of a system.

In some engraving styles (mainly jazz), a double system initial bar line is used at structural articulations of the piece.

The PR supports this style, by adding a `_systemInitialBarLineStyle` variable to the `Measure` class which controls the style of the system bar line when the measure is the first of the system and by default it is set to NORMAL. This variable is not accessed directly; rather it is controlled by manipulating the system bar line. A system bar line can be edited:

- by dropping on it a bar line style from the palette (structural styles, like any repeat or END are ignored);
- by selecting it and double clicking a bar line style from the palette (same note as above).

It can be reset:

- by undo;
- by dropping on it (or selecting and double clicking) a NORMAL bar line style from the palette;
- by selecting and deleting it.

As the system bar line style is stored in the measure, if the first measure of a system changes (because of some other editing), the system bar line style will follow accordingly.
fix #32651: Naturals should not be shown at system break
fix #39731: Stop midi flooding
fix #6535: grace note ties, articulations, and undo fixes
add vtest
add mtest
Fix tab duplication when 'Apply' pushed in the MIDI import panel
Fix addition of imported MIDI files to the recent files list
Fix duplicate score name
Remove #ifdef because the 'if' branches become equal
Don't add recent score if it is already last in recent list
Fix empty recent file path: save file before add it to recent list
Fix last session MIDI file path
fix #37376: Crash when paging through file containing multi-measure rest
fix #15629: Mac: Play Panel doesn't float above main window
Merge pull request #1487 from MarcSabatella/6535-tie-grace

fix #6535: grace note ties, also articulations & duration fixes
fix #40356: lower case minor chord issues
fix 40481: Page Settings spinbox step size too large in inch mode
Merge pull request #1324 from CombatCube/32651-naturals-system-break

fix #32651: Key signature at system break with canceling naturals should not repeat naturals at beginning of following system
fix #23137: Beam of small chord aren't small