[kfontinst] Port to use icons from the icon theme and make HiDPI aware

Summary:
Use QIcon instead of QPixmap for icons. Use QPushButton for the filter menu instead of a QLabel.

{F7340233}

Depends on D23942

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23868