Remove a conversion to_utf8() inside FontSetChanger

Remove a conversion to_utf8() inside FontSetChanger

This requires to change many docstrings into std::strings. The logic behind that
is that they represent a fixed set of math fonts, and therefore “string” means
here “poor man's enum” rather than text (this is consistent with MetricsBase).

Profiling of scrolling inside a document over macro-instensive areas:

Before the patch:
  44,1% BufferView::updateMetrics()
   -> 34,8% InsetMathHull::metrics()
     -> 9,8% FontSetChanger::FontSetChanger()
  28,4% BufferView::draw()

After the patch:
  35,3% BufferView::updateMetrics()
   -> 27,2% InsetMathHull::metrics
     -> 0,4% FontSetChanger::FontSetChanger()
  47,5% BufferView::draw()

FontSetChanger::FontSetChanger() is made 41x less expensive (with reference
BV::draw()) just by removing this conversion. The remaining 0,4% could be
squished by replacing the strings with a proper enum, but this is premature. Of
course, this only treats the symptoms: there is no good reason that this
function is called 45500 times over the time of 40 repaints.