Fixed visible wrap column on systems with non-fractional fonts sizes

QFontMetricsF::averageCharWidth seems to always return a fractional size
(at least on Linux/GNOME with DejaVu Mono), even when the font is not
actually drawn at sub-pixels. This caused the visible wrap column to
appear in the wrong place.

As a workaround, use the width of the character 'x', which is pretty
average and does align to pixels when needed.

Task-number: QTCREATORBUG-2746
Reviewed-by: con