[src/] port more code away from kdelibs4support and other deprecated classes

Summary:
Drop setOpaqueResize() call in konqframecontainer, let the style use
its defaults.

Remove some spurious #include's

Don't setFont() on widgets, let the widget/style handle it

Reviewers: dfaure, stefanocrocco

Reviewed By: dfaure

Subscribers: ognarb

Maniphest Tasks: T11559

Differential Revision: https://phabricator.kde.org/D24791