[KFontChooser] More code cleanup

Summary:

• Simplfy the layout by removing the splitter, the goal was to allow resizing the font preview text edit, which seems like an unused feature (I didn't even know that was a splitter until I read the code)
• Remove redundant tooltips and whatisthis, IMHO it's self-explanatory
• Move most code out of ctor to an init() method, less d-pointer redirection (and slightly more q pointer redirection)
• Change the maximum font size from 999 to 512, which is what QFontDialog uses, personally I think neither value is realistic, but I am keeping an open mind (8K monitors?)

Test Plan: kfontchooerdialogtest still works

Reviewers: Frameworks, dfaure, cfeck, apol, bport

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28271