Implements melisma and dash lines for lyrics spanning several systems.
The melisma and dash line is based on the `SLine` class and its segments on the `LineSegment` class. Both the whole line and its segments are not selectable, marked as generated and not saved in the score file, which is not changed in any way.
For very wide dash segments, several dashes are drawn; the distance between the dashes is not configurable.
Lyrics layout code in `Measure` class and in `layout.cpp` file has been commented out as the lyrics line layout is all contained in the lyrics.cpp file
The line is registered with the `Score` (to have its layout delayed until all elements are positioned) with a mechanism similar to other `Spanner`'s, but in a different container (`_unmanagedSpanner`), as the owning `Lyrics` should decide when create, register, unregister and delete its line.
The line segments are registered with the `System` they belong to (to have them drawn), in the same way as other `Spanner`'s.
There is code for using the dash metrics of the lyrics font, but it is turned off via a conditional directive, as there does not seem to be a reliable way to determine the dash metrics; conventional values (determined by trials and errors and based on my taste!) are used when the conditional directive is off.
reformatting, removing some comments that are irrelevant to plugin
framework in a way that DoxyGen should still find them, documenting the
enums and fully qualify all types that are specific to MuseScore,
