Buttons: allow to scale up icons

Summary:
always scale icons unless a size is specified in the api.
this makes icon sizing more in line with plasma components 2

Test Plan:

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27628