Bar lines: fixing custom type and generated management

There are some inconsistencies in the current management of bar line `_generated` flag and user-modified type:

- Bar lines created by the New Score Wizard are flagged as non-generated, as well as bar lines of measures manually **appended** by the user, while bar lines of measures **inserted** are flagged as generated.
- If a generated bar line is individually changed of typed, it remains flagged as generated, it is not saved and the change is lost upon saving and re-loading.
- The management of the internal flag `BarLine::_customSubtype` is not always consistent.
- The `Measure::_endBarLineGenerated` flag was not always restored properly by undo.

This PR introduces the following fixes:

- The `_generated` flag is consistently used for bar lines whose type can be reconstructed from the context and then do not need to be saved to the output file.
- Normal bar lines are **always** created as generated: initially created by the Wizard, manually appended or inserted.
- Bar lines with custom type (i.e. different from the type which can be expected according to the bar line context) are always flagged as non-generated, ensuring the custom type is written to the output file.
- The `Measure::_endBarLineGenerated` flag is stored by `ChangeEndBarLineType()` and restore upon undo.
- Some test reference scores, based on the inconsistent bar line `_generated` flag, have been uniformed.

Notes:

- Tests about measure (and then bar line) appending, inserting and relative undo's are already included in the `tst_parts` test suite.
- Some inconsistencies remain in the management of custom bar line span and of system-initial bar lines: for the sake of simplicity, they will be dealt with in separate PR's.
Add default value for BarLine::updateGenerated()
Merge pull request #1281 from mgavioli/barline_custom_type_and_generated

Bar lines: fixing custom type and generated management