Fix pixelated icon scaling for HiDPI screens

Summary:
This enables HiDPI pixmaps in the main window. This commit also replaces QPixmap and KIconLoader with QIcon to load properly scaled icons in the tree menu for HiDPI screens.

BUG: 390737

Before:

After:

Test Plan:
Open icon selection in main window.
Expand tree menu and ensure selection works.

Reviewers: rkflx, broulik, davidedmundson

Reviewed By: rkflx, davidedmundson

Subscribers: davidedmundson, cfeck, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11631