Work around issues with Qt5 and Arabic text

Work around issues with Qt5 and Arabic text

This fixes two particular problems

* with Qt5, it seems that QFontMetrics::width does not return the
  correct value for some Arabic text; this patch uses QTextLayout
  instead to compute a string width

* Likewise, the undocumented layout flags TextForceRightToLeft and
  TextForceLeftToRight do not work with Arabic text; this patch uses
  unicode override characters instead.

It might be that the two issues are related. In any case, they do not
happen with latin text where right-to-left direction is enforced. And
they do not happen with Qt4.

Additionally, remove some dead code in GuiFontMetrics::pos2x().

Fixes bug #10436.