Sets form's target to `_blank` when submitted while Control key is being pressed. Allow submit with middle mouse button.
Per
Useful for "saving" form state in preceding tab.
Ctrl+enter will work alike where plain enter would submit form (ie NOT on textareas).
Middle mouse button click on submit buttons should work as well, just for the basic structure and with many unhandled border cases. And probably with different focus management compared to Ctrl + Enter.
For reference, request for similar functionality in Firefox from 1999.
