Work around broken math symbol display (bug 7954)

Work around broken math symbol display (bug 7954)

This extends the already existing math symbol fallback mechanism in two ways:
1) When considering the availability of the math font, also take broken
   code points into account. These are currently 0x0009 and 0x00ad, depending
   on the platform.
2) If the fallback symbol in the standard "Symbol" font is not given, or if
   the "Symbol" font is not available, or the fallback symbol is one of the
   broken ones, try to use a generic unicode symbol as second fallback instead.
   If this is available, we rely on Qt to find a font which has it. Only if
   this is not available, display the symbol as ERT.
This ensures that we do never get a symbol which is not displayed: Either
it can be displayed, with or without fallback, or it will be shown as ERT.