iconGrid: Stop using Shell.GenericContainer

iconGrid: Stop using Shell.GenericContainer

Removing Shell.GenericContainer from the IconGrid class was
challenging because it needs the "skip paint" API from it.
This API was added, too, as a workaround to the inability
to override vfuncs from GJS.

The overrides are largely copy-pasted and translated versions
of the Shell.GenericContainer code.

The IconGrid:key-focus-in signal was renamed to :child-focused
to avoid clashing with ClutterActor:key-focus-in.

In GridSearchResults, the internal IconGrid had it's y_expand
set to false, so it doesn't push other search elements (the
list results mainly) to the bottom of the screen.

Because skip paint wasn't and still isn't a GObject property,
rename it to _skipPaint to reflect that.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/153