HiDPI icon pixmap

Summary:
Return HiDPI icon pixmap, so the icon in process list is sharp in HiDPI display.

Before:

{F7331534}

After:

{F7331521}

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: GB_2, #plasma, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23794