Port kcm icons to kconfigxt

Summary:
Also fix 2 bugs:
- keyboard navigation on size category didn't work as expected, we didn't saved what we see
(open size widget: arrow down, arrow right, save => will save the size to desktop entry instead of toolbar
- the icon slider was not updated when we change theme

Reviewers: ervin, mart, #plasma, crossi

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24846