Fix #281639: Cutaway staves - better handling of clefs, barlines...

[elerouxx]

Resolves: https://musescore.org/en/node/281639

Better handling of clefs and barlines for cutaway engraving and ossias.

Barlines:

• Begin Bar Lines now can be added to a measure by adding to the first chordRest of the measure
• A measure Begin bar line will overlap previous measure's end bar line so there is no visual change if both barlines are visible
• The span handles of the Begin bar line are slightly at right of the barline
• the span of this or other custom (added) bar lines don't set the global span anymore.

(ossias with added start barlines)

Clefs:

• A clef present on a measure makes the measure visible
• As usual, clefs added to the whole measure show before the bar line, thus making the previous measure visible.
• Hidden clefs can be used as a workaround to force unhide a measure in cutaway staff (or a whole staff when staves set to hide when empty).

Clefs (Update):

• In cutaway staves, clefs added before barlines on an empty measure (as in adding a clef to the the the next measure) turns the measure visible but showing only portion of its staff lines lines, as a clef "trailer" (or "leader").
• (if desired, rests need to be hidden manually).

*A workaround to force measures to be non-empty (and force visible) is to use a staff text, set to white or transparent, since the measure containing the text can be easily copied and pasted around.

*(Clefs added to measures partially reveal previous measures as courtesy/trailer for clefs. )

Brackets: will be left out of the scope of this PR.
Any feedback is appreciated.

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• I created the test (mtest, vtest, script test) to verify the changes I made

[elerouxx]

I wrote this function lines()->layoutPartial() (maybe need a better name) that draws only a certain width of the lines and aligns them to the left or right of a measure. In this specific case I needed just enough lines width to fit any Clef.

On a related topic, I have been looking for a way to overcome MuseScore's limitation of having a staff change in the middle of a measure, since the staff lines are drawn per measure. But I believe this function might give a clue of how that could be done.
The code above would:

• Loop thru the staffTypes in the measure
• Keep appending lines to ms->lines, calculating their widths (and yOffsets, etc)
• Example: if there is a staff change from 5 to 1 line in the middle of the measure:, 6 lines will be added to mslines: 5 from the beginning of the measure to the tick of the stchange, and one from there to the end of the measure width.

[Jojo-Schmitz]

Please check the mtest failures at https://travis-ci.org/github/musescore/MuseScore/jobs/717104942#L4705-L4769

[elerouxx]

I found a better solution, with a far less messy workflow from the user side.
#6461

(Closing this PR)