Fix checkbox inconsistency: label [ ] -> [ ] label

This commit fixes some checkboxes that had their label to the left to add some consistency.

While at it, I also added label tags, so the labels can be clicked to (de)activate their checkboxes/radios.

Checkboxes concerned:
"Display reference lines" in Navigator
"RGB slices" in crop tool
4 checkboxes in classification editor
Merge pull request #636 from fzadow/fix-checkboxes

Fix checkbox inconsistency: label [ ] -> [ ] label