ContextualHelpButton: Fix flickering when the popup covers the button

On smaller windows, the toolTip popup can cover the contextual button, so the hover condition is lost and the toolTip closes. This makes the button get hovered again, which results in a continuous open/close loop

Let's keep the tooltip open also when the popup itself is hovered.

BUG: 489688
FIXED-IN: 6.6

I've only been able to reproduce this on the Window Rules KCM, which also has the longest tooltips.

BEFORE	AFTER
chb-popup-flicker	chb-popup-fixed

- Clip Content

While at it, on a second commit, clip the content so it doesn't overflow the background. The text overflowing was easier to notice on this circumstance and when the flicker is gone.