Time sig property editor: allows more symbols to be entered in the user-defined texts for numer. and denomin.

Allows for 'C', '¢', 'O' and 'Ø' to be entered in the text lines for numerator and denominator in addition to digits.

This make possible and easy to enter Renaissance or early Baroque time signatures like 'C3' or 'Ø3'.

The usefulness of course is limited to a few niche users, but should not harm the other in any way. the full repertoire of Renaissance and Baroque time sigs is still not covered, as it includes time sigs like "C3/2" or "O6/4" which will be possible with the (future) more extended time sig editor which will allow modern composite time sigs.
Replace non-ASCII chars with hex representations.

(Also added a few comments about glyphs for less common time sigs)
Merge pull request #810 from mgavioli/timesigprop_user_text

Time sig property editor: allows more chars to be entered in the user-defined texts