[KWindowSystem] in icon() return realistic icon size

Summary:
The icon function did not take into account the fact that icons have limited size options available.
Also take into account scale parameter.

Reviewers: #kwin, davidedmundson, zzag

Reviewed By: #kwin, zzag

Subscribers: anthonyfieroni, mart, zzag, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24683