[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