This is my first question. I have tried the search and not managed to turn up exactly what I'm after, so here goes:

If I have a shape, any shape, or an image clipped with any shape (as long as the shape has at least one straight line segment): Is there an easy way to rotate the shape such that a chosen edge is vertical/horizontal?

I can obviously do this by inspection, but then I can't really be sure that I've got it perfectly aligned.

My actual situation is that I have clipped an image with a rectangle that is at an angle. Now I want to make the rectangle perfectly aligned with horizontal/vertical. I could go back and start with an aligned rectangle and rotate the image until it's at the right angle, but I feel like I ought to be able to do it this way round.

I would probably move the center of rotation to a corner and snap the corner to a guide. Then rotate the rectangle until another corner snaps to the guide.

An "ugly" solution idea:

rectangles have their transformation values saved in the file. With the xml editor you can copy the transformation matrix,
which you need to make an inverse matrix of and apply it on the clipped object.

If only rotation was involved you can flip the clipped object, apply the rectangle's transformation matrix on the clipped (and mirrored) object, than flip it back.