terminal: get the pid from GCredentials on 2.35.4+

The GJS 'system' module is going away and the only introspectable API to
get the pid of the current process is in GCredentials.
Updated ROADMAP
Added support for gnome-shell 3.7.4
Updated the README
Updated the ROADMAP
Added support for 3.7.5
Misc. cleanups
Merge upstream changes.