* 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().