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 styles 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.