Bar lines: Fix custom span and generated management.

This PR should complete the revision of bar line flag managements.

- Make sure single bar line span changes affect both the `_customSpan` and the `_generated` flags.
- Make sure that manually bringing a custom-spanned bar line to default span resets the `_customSpan` flag and, if no other customization is in effect, the `_generated` flag.
- Deleting a measure bar line resets it to default configuration.
- Fix a missing initialization of `Measure::_endBarLineColor` variable.
- To simplify tests and debug, check boxes for the `BarLine::_customSpan` and the `BarLine::_customSubtype` have been added to the debugger dialogue box.

As far as system-initial bar lines are concerned, they were made un-editable by a recent commit (https://github.com/musescore/MuseScore/pull/1300). This PR add a few consistency changes:

- system-initial bar lines do not accept drops;
- they are not saved to score output files and are ignored when reading from them;
- their internal `_customSybtype` and `_customSpan` flags are always false;
- they do not show up in the Inspector (if a system bar line is selected, the Inspector remains blank)

This PR DOES NOT include the special system-initial double bar management recently discussed. This will be part of a specific PR in the next days.

Together with previous PR's, this PR should ensure that:

- bar lines are written to a score output file only when some customization is in effect which cannot be reconstructed when reading back;
- any measure bar line can be edited;
- any measure bar line user edit is saved, written to the score file and read back properly;
- no system bar line edit is possible.