Add pixel value under cursor to statusbar.

Also fixes corner-case of (0,0) coordinates.

I'm pretty sure the way I extract the pixel value is not the best one. I played around with the `ImageSlice2d` but failed getting it from there (The `imslice` member is only the currently displayed subarray.)

The `vec(val)` part is there for m-n-3 images, where `val` would be a column vector, and the `length(val)==1` guard for grayscale images. I couldn't test for 3D/4D images since I don't have any.