How to slide a vertex with UV to avoid stretching? I would like to move some vertices of a plane created through the "import image as plane", but without stretching the texture. A kind of crop like in libreoffice writer. 

Side-notes: 

Best would be a trunk function but if their is no such function, a way to move the vertex in both 3D view and UV editor (an add-on or with some conversion maybe ? Like 3Dview_vertex_move(x,y)*(a factor)=UV_vertex_move(x2,y2) ) at the same time would be nice.

I know the knife tool can do the job somehow, but it involves many more steps (deleting old vertices, etc...). I'm looking for an efficient solution as I have to do it a lot.
I'm not sure about programming it in, but here is how I would do it.
After importing the image go ahead and put in your loop cuts or whatever where you want to crop the image to.

This distorts the image. To fix that, just re-unwrap it.
The command is bpy.ops.uv.unwrap()
Or you can hit W > Unwrap.

Then just delete the outside vertices and you will get a cropped image.