export paper model: fix bugs in UI and export (upstream edc29e0)

* Fix `Mesh.copy_freestyle_marks` to properly read from Mesh modified in Edit Mode;
* Add UI panels to 3D View, in own category;
* Report an error if exporting object with negative scale;
* tidy up `Mesh.check_correct`.

Reviewed By: dfelinto

Differential Revision: https://developer.blender.org/D6232