Restricts accepted characters for time sig. numerator and denominator to '+', '(', ')' and 0-9 in Time Sig. Properties dialogue box.
More characters for time sigs:

Code:

* glyphs for '+', '(' and ')' registered for time sigs. added to MScore (in the same code points as SMuFL)
* glyphs for prolationes defined in SMuFL added to MScore registered for time sigs. (in the same code ponts as SMuFL; some existed, but in different code points)
* transcoding ASCII -> SMuFL for '+', '(' and ')' added to Element::toTimeSigString()

UI (not final for font problems):

* In Time Sig Properties dlg box, converted "Common" and "Alla Breve" strings to corresponding symbols
* UI for input of prolationes added to Time Sig Properties dlg box.

Also:

* Fixed calculation of Time Sig bounding box (also fixes horiz. alignment of numerator and denominator with different width).
Time Sig Props dlg box layout

Added icons to display time syg symbols and changed layout to two columns of "Values" and "Appearance".
Merge pull request #569 from mgavioli/time_sig_texts

Time sigs: update glyphs and UI to SMuFL