Feature: Rotation

[KristofferStrube]

It would be nice if we could rotate shapes. We can do this using CSS transforms but we should make sure to scope this feature as it can get rather complex if we have multiple series of rotation combined with scaling and translation.

There are multiple possibilities for scaling. The best one would be to have some natural mapping like a third anchor that can be used explicitly for rotation. Other simpler ideas could be to have discrete options for rotation like "Rotate 90 degrees" or to have some input in the context menu to control the rotation numerically.