ApplicationPage: Fix excessive horizontal flicking

Explicitly set the preferred layout width for the selectable labels to the actual width after wrapping (contentWidth), instead of the default implicitWidth.

For selectable labels, the implicitWidth is the full width of the text before wrapping. When his value is big enough, it bubbles up the layout chain and ends up allowing to horizontally scroll the view to the right.

There is no binding loop, since the actual width calculation already uses fillWidth.

For testing, the effect is more noticeable the longer the longer description paragraphs (GIMP is a good test case), and horizontal scrolling might also require a touchpad.

BEFORE	AFTER