Fix indentation of icon for left aligned toolbuttons

Summary:
Gwenview's Operations sidebar uses custom QToolButtons which are
wide and left aligned. Special support for this in Breeze was added with
2fb62271d777. However, compared to the KDE4 version there is not enough space
between the left button border and the icon:

This is fixed by adding the appropriate margin:

Note how this replicates exactly the spacing of the toolbuttons in the
regular toolbar on the top (which had its icon size set to "small" in
this screenshot).

CCBUG: 381535

Test Plan:
Icons on toolbuttons in Gwenview's Operations sidebar no longer
squished all the way to the left, text spacing still looking good.

Reviewers: hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9281