[kcms/fonts] (Re-)Add tooltips using a contextual help button

This commit adds tooltips to font settings for which the label alone isn't sufficient for laymans to understand their effect. Similar explanations were previously available in the form of "What's This?"-help but got lost in commit 24b960f9: "QML port of fonts kcm" perhaps because tooltip functionality was not readily available at the time. The tooltips added here are partly based on these older ones.

The tooltips are invoked by either hovering or clicking flat Contextual Help Buttons. This way the available help is unintrusively visible and can be easily invoked on touch devices.

First time using QML so expect noob mistakes everywhere!

2020-06-28_19-04-36 (Tooltips in this video are up to date except one word: I added a "the" in the last tooltip right before "real DPI".)

I think these kinds of buttons might also be the best way forward for a lot of other settings pages because they have some usability advantages over "What's This"-help and are not as intrusive as always visible text. Related: https://phabricator.kde.org/T9986