Add 32, 48 and 64 px user-desktop icons

Summary: This prevents the icon from becoming blurry at fairly normal panel widths.

Test Plan:

• make/ninja install
• rm -r ~/.cache/plasma*
• plasmashell --replace &

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25897