main_window cleanup

[Daniel J Sebald]

I put a changeset to clean up the main_window conditional construction here:

https://savannah.gnu.org/bugs/?44116

It has lots of hunks, but basically it goes back to a prior version but 
moves the link construction outside of main_window and supplies the 
needed shutdown_confirmation and exit_app signals/slots for --no-gui 
mode that otherwise would be provided by main_window in normal mode.  In 
the former case they are very simple.

Dan