The gesture internally manipulates the main adjustment so one swipe
up brings up the overview, and a second swipe up brings the app
grid. The gesture also works in the other direction to get out of
the overview.
Internally, this is delegated on the OverviewControls, so the
adjustment is not leaked out of there. This however meant open
coding the gesture interaction so it can be directed from
overview.js code.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1643>
