Faces should be exported with shared vertices whenever possible (otherwise this is simply a STL exporter). Since PLY stores normals per vertex it's understandable that we need 'individual' faces when exporting normals. However when this is not the case I think shared vertices should be default behaviour. This is what this patch implement.

Thanks Campbell for reviewing it. Thanks Aldo Zang for reporting it (off trackers, in the old fashion real life)

update addons enabled by default to use 'faces -> tessfaces' also grease pencil scatter and quake map export.

.PLY has no specification for how to represent the texture coordinates.
The general guideline is to implement this the way is more convenient for the software (this it makes no sense to flip what Blender does internally).

Also, there are some softwares that used to flip V in the past for .ply. This has changed though, and from what I could find in the internet, the current standard is to have the V opposite to what we have in Blender.

Finally I used meshlab for my tests - http://meshlab.sourceforge.net/
Their sample files (the dount one) shows flipped V in blender.

I hope no one gets unhappy with this commit.

thanks Campbell for review and tweaks

Adding UNDO flag to import operators. Made this only for trunk addons.

- remove/comment unused variables
- remove unused imports
- fixed some bugs using incorrect variables

update_tag() tags for depsgraph update only, update() executes normal recalculation and creates edge data.

most editors (besides notepad) will open these files without trouble.

- Display the right docstring for ply files
- Add a filter to *.stl on importing stl files