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.
Merge pull request #1316 from mgavioli/system_initial_barline_style

Style of the initial bar line of a system.