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 #36186 : crash when pasting with left-over portion spanning over an invalid duration
fix #39621: accessible info for drum and slash notation
fix #39461
	modified:   mscore/drumtools.cpp
native 'break every X measures' function
Merge pull request #1466 from andreituicu/fix_39461

fix #39461
Merge pull request #1465 from MarcSabatella/39621-accessible-nonpitched

fix #39621: accessible info for drum and slash notation
select first score on default in startcenter scorebrowser
Merge pull request #1462 from AntonioBL/second_voice_split

fix #36186 : crash when pasting with left-over portion spanning over an ...
add test for fix #36186
Merge pull request #1467 from MarcSabatella/native-breaks

native 'break every X measures' function
fix to plural
Merge pull request #1468 from Jojo-Schmitz/pre-translation

fix to plural
fix #39641: ctrl+c does not work in screenshot mode
fix warning reg. unused parameter
implement copy&paste for image mode
dont allow drag of all elements if measure is selected
Merge pull request #1469 from Jojo-Schmitz/warnings

fix warning reg. unused parameter
fix build