Fix changing of anchor properties

This also makes it possible to undo/redo the changing of anchoring of shape.
It makes sure that all is done inside a command so that it is undoable.

This also fixes a crash that happens when you do the following:

- Open a blank document
- Type some text
- Insert a rectangle.
- Make it anchored to paragraph
- Undo until the rectange is removed
- Insert a new rectangle.
- Anchor it to the paragraph.
-> crash