portalHelper: Add basic zoom support

portalHelper: Add basic zoom support

A portal login page may have poor readability that require page zoom
functionality as implemented by most browsers.

However WebView does not provide this functionality itself, it only
exposes API that allows implementing it.

Add basic zoom in/out/reset actions based on epiphany's implementation,
and hook them up to accels.

Exposing them more prominently (as buttons or in a menu) would require
some UI rework, so leave that for a later.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/9129
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4159>