Rely on Qt to handle small caps text

This has the advantage of simplifying our code and to produce the
correct output: the small capitals should have the exact same width as
the lower case letters.

The slanted fonts are also translated to oblique on Qt side, but this
does not seems to have an effect in my testing. It may be that proper
oblique fonts need to be installed.
Listings: Validate skip amounts correctly.

Fixes: #7373.
Fix problem with drawing of math backround with selections.
Monolingualization