kcms/smserver: use a ContextualHelpButton to show the tooltip
requested to merge ngraham/plasma-desktop:use-contextualhelpbutton-instead-of-tooltip-for-ksmserver-kcm-application-list into master
Right now to see this tooltip, you have to hover the cursor over the text field, which is not terribly obvious and not possible with touch.
Instead, let's use an always-visible ContextualHelpButton, which was created for exactly this use case.
