Fix relative transformation actions

This is a regression that happened a long time ago when we moved to actions.

Previously inkscape allowed transforms to be anchored against whichever anchor was being hovered over. Today, we have a click to select UX instead. But this did not carry over to the transformations which are driven through g-actions.

Refactoring:

• setScaleRelative is renamed scaleRelative to make it consistant with the other transformation relative functions.
• Action specific functions moved from object-set, to the actions code itself removing a few uses of the desktop object in the object-set.
• Functions added to get the desired anchor point from the selection easily.
• Move screen zoom based actions to window actions so we don't have dangling app actions that don't work for the command line.
• Remove rotate keys from the selection-tool and turn into actions like the scale operators.

Fixes #5456 (closed)