make the sizing of the toolbuttons more coherent

Summary:
this makes the toolbutton icons follow a standard size.
Maybe one could make it follow the icon group size props? like icon.width and icon.height if those props are set.

Before patch:

After patch:

Reviewers: Plasma, mart, ngraham

Reviewed By: ngraham

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23367