I do understand your request for this function. But PM is not an image editor, and hence perhaps not the right place for things like this.
I mean, the problem is, where to stop with adding these types of editing functions?
By the way, the best implementation of straightening horizons etc. is perhaps the one found in Nikon's Capture NX. You simply draw a line that should be straight in the image (e.g., alongside a window sill, doorpost, horizon, etc.) and it will take care of the straightening immediately, optionally cropping the areas without image data.
Much easier than the way it is done in Photoshop (where you first have to measure the angle), and Lightroom/ACR (where the image isn't rotated, only the crop area). Plus, the NX version works both with horizontal and vertical lines!
Cheers.