Expose font substitution to user

MuseScore 1.2 and current dc5679f8e0

1. A user create a score with a title using a given font
2. The user opens the score on another computer where the font is not present

Currently, MuseScore substitutes the font silently by the closest font, using Qt font fallback mechanism. It would be good to expose this mechanism to the user and let him choose which font he wants to use, or rely on the automatic matching. Here is how Sibelius exposes it to the users. Note that there is an option to substitute the font while drawing the score, but keeping the original font name. This should allow an edit, without loosing the font information.