Bring back usericon region selection
Summary: Also, improve error checking.
Test Plan:
- selected an new icon, got the region selector
- without the resize, saving will fail for high resolution images
- When resizing to a bigger size (e.g. 800x800 pixel), icons will be too big
- Canceling the region selection will not change the icon
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D3134