Provide magic menu checkbox for pasting magics instead of direct executing

This is related to issue #2565 (https://github.com/ipython/ipython/issues/2565).
There is no reasonable way to programmatically distinguish between magics with parameters
and magics that do not require params.
Implemented "Only Paste Magics" menu checkbox. When checked menu items will just paste
magics into the input without executing them. When unchecked (default) magics are executed.