Extensions can have a major impact on stability and performance. Now that
the tool will become the main way for users to manage their extensions, it
is an appropriate place to warn the user of that risk.
Add a small info popover to the headerbar to display that warning, together
with the previously removed hint of where to go for finding new extensions.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1968
Currently when the extensions list is scrolled, it is possible to
keynav out of view, as the scrolling doesn't follow the key focus.
Hook up the adjustment to fix that.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1968
