global: Add magnifier activation property and bind to settings
The same code for reading the current magnifier state is repeated in both shell-recorder, shell-screenshot and magnifier itself. So to move this inside a property of shell-global so that we can refer to it all over the places removing duplications.
No need to keep track of the gsettings object, because binding it to the global object, automatically implies that the binding adds a reference on it that is keept alive until the global object itself is valid.