Revert "[showdesktop][minimizeall] Reduce the maximum panel icon size"

Summary:
This reverts the following commits:
fff556af
5de082c3
33be0f5b

I think I did this change the wrong way. What we really need is a way to use different sizes for different panel widgets by default without making it impossible for users to configure the size.

Reviewers: #vdg, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22359