[KFontChooser] General code cleanup

Summary:

• Use new connect syntax
• Prefix increment where appropriate
• Replace foreach/iterator-based for with range-for
• Remove (really) old TODOs

Test Plan:
make && ctest
KFontChooser in e.g. Kate still loads and works as before

Reviewers: Frameworks, cfeck, apol, bport, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28191