TAB - Mixing mensural value symbols and beaming in historic tablatures

[mgavioli]

References: Technology Preview forum post with discussion, screen-shots and links to additional threads https://musescore.org/en/node/81051

Implements the possibility to mix, in TAB's with note symbols at staff side, mensural value symbols (discreet glyphs) with the 'grid'-shaped beaming found in historical sources and commonly used in lute literature.

The beaming is implemented by special drawing of the TabDurationSymbol element holding the note value symbol, using drawing primitives instead of the relevant font glyph.

The user may choose between the two renderings (discreet glyph or beaming grid), on a chord-by-chord basis, by setting the chord BeamMode (via for instance the relevant palette): AUTO selects the glyph rendering, beam start the start of the grid and beam middle the continuation of the grid beamed to the previous element.

Typographic features of the grid (stem width, stem height and beam thickness) depend on the glyph style and are hard-coded in the style definition. As well as the number of beams for note value, which also depends on the note value style.

Also:

• Implements to possibility to force the display of a note value which would not be rendered by the current note value repetition setting, by setting the chord beam mode to any other value.
• Implements the 'no stem' chord setting for this TAB style, allowing to remove a note value symbol otherwise generated by the current repetition setting.
• Improves the detection of note value font metrics (still not perfect, though, as Qt QFontMetricsF::tightboundingRect() returns very approximated results)
• Fixes note value glyph scaling, when the staff scale is modified.

[mgavioli]

@lasconic @wschweer : would it make sense to merge this PR to 2.0.3 too? This feature is practically self-contained and does not affect the file structure; if a 2.0.3 score using it would be read by 2.0.2, this feature would be simply ignored.