Default style, base style, saving and reading score file styles.

Background: the MScore objects has two reference styles:
- _baseStyle, which is inited to the built-in default style (common to all MuseScore copies)
- _defaultStyle which is inited to the user-defined default style
They are equal in a default installation but become different as soon as the user chooses a custom default style in the Preferences.

Issues:
- score style are saved as delta from the user-default style for texts and for articulations and as delta from the built-in default for the rest
- when the score is read back from file, it is inited to the user-default style and styles are read as a delta from it.

This makes:
- different parts of the saved style inconsistent among themselves
- styles saved in the score non-portable across different MuseScore copies (with potentially different user-default styles)

Fix:
- all style parts are saved as delta from the built-in default (to be portable)
- score is inited to built-in default style before being read from file.

Note: it is expected that users with custom default styles notice style differences when reading scores saved with previous 2.0 commits, as they were saved as delta from their respective user default styles.
Fix #6107 : bar lines and brackets disappearing when some of the staff they span is hidden

Fixed by analyzing brackets and bar line spans rather than only relying on start and stop staff Y offset (which is not set if the staff is not shown).

Note: there are still issues with measure numbers, but these belong more to system items than to bar lining.
Add test
Merge pull request #604 from mgavioli/Fix_6107_Barlines_with_hidden_staves

Fix #6107 : bar lines and brackets with hidden staves
Merge pull request #602 from mgavioli/basestyle_and_defaultstyle

Default style, base style, saving and reading score file styles.