fix #29246: unrecognized chords with symbols imported with markup

I elected to fix this within the Text class as it seems the same fix could be useful elsewhere.  The fix is, for Text items that contain accidentalFlat or accidentalSharp symbols, when calling plainText(noSym = true), instead of skipping those symbols entirely as we would for other symbols, I replace them with "b" and "#".

Right now, it looks like the only other place plainText(true) is used is in generating filenames for newly created scores from titles.  So if you set the title of your score to "Concerto for Bb Trumpet" but with a real flat sign, this would substitute a "b" for the flat sign in the generated filename rather than leave it out entirely - a good thing, I would think.