Adopt a 'belt and braces' approach to bidi forcing

Adopt a 'belt and braces' approach to bidi forcing

There are two techniques that I know of for forcing the direction of a
string, regardlessly of whether its contents is naturally LtR, RtL or
undecided.

1/ The unicode LTR/LTR override characters. This is supposed to be the
   clean way, however, it does not seem to work with Qt 5.14 (see
   #11691).

2/ The undocumented QTextLayout::setFlags method. This is used
   internally and allows to pass the (undocumented) flags
   Qt::TextForceRightToLeft and Qt::TextForceLeftToRight. This was
   used until we had issues with Qt 5.11 (see #11284).

In order to get the best of both worlds, this patch allows to enable
those two methods separately, and actually enables both at the same
time by default!

(hopefully) Fixes bug #11691.