Print3D: use subpanels to declutter UI

Print3D has three tool categories: Mesh Check, Cleanup & Modify, Export. The first category being the main point of the add-on, the rest are supplementary and should not take visual space by default. Notes: Ideally Cleanup & Modify should be their own categories, but there are only two Modify tools so I decided to merge into one category. There is a point to make that Cleanup & Modify panel should be open by default, I am not strongly agains that and it could be done upon receiving feedback from users. Also fix multiple Select Non Manifod operators displaying in report.