Lock printable area and common elements

[pscanf]

https://user-images.githubusercontent.com/3725458/140566867-b86ae706-cbf6-4b12-bb36-e6af4d460fc1.mov

At the moment the user can freely interact-with and modify the printable area element, and the common elements (in Slides view, obviously modifying common elements in Settings view is ok).

This is not a big deal, since these elements are not saved into slide elements, and are re-injected every time the user switches to a new slide. It would be nice however to "lock" them, so that the user cannot accidentally interact with them.

Note: follow / contribute to https://github.com/excalidraw/excalidraw/issues/1976