Bugfixes: Copy/Paste rendering settings

This is the same as #3117, just manually rebased (cherry-picked) to current develop (will close #3117, couldn't get it to compile anymore). Hopefully this will fix the outstanding issues with rendering settings copy/paste.
To test: Go through the Copy/Paste rendering settings scenario.

There is one weird minor layout issue left: For images with big possible channel values (i. e. channel min/max values >= 5 digits) the text fields tend to be too big. Don't want to spend hours with fixing that, but maybe I'll have a sudden inspiration and can fix this within this PR, too.