Work around bug in QTextLine::xToCursor

Work around bug in QTextLine::xToCursor

With RtL text, the method can be non-monotonic wrt x value (which is a
Qt bug). To work around this, we check whether positions adjacent to
those returned by xToCursor look better. Depending on whether the new
x position is too small or too large, we look backward or forward for
a better solution.

The performance is probably not great, but this is only needed for
user interactions, so the performance penalty should not be a problem.

Fixes #10569.