improv: Allow reflowing for narrow width

[ids1024]

As suggested on pop-os/gnome-control-center#202.

Will require update to desktop widget with this version to apply on hirsute/impish.

[jacobgkau]

This is causing the Appearance page in GNOME Initial Setup to be too large vertically:

Can't test the actual reflowing behavior since GNOME Initial Setup does not resize (and desktop-widget in Settings is not using this branch.)

[ids1024]

(and desktop-widget in Settings is not using this branch.)

If you test with pop-os/desktop-widget#73, it pulls in this version.

Doesn't seem to have that issue in Gnome Control Center. Not sure how Gnome Initial Setup differs...

[ids1024]

Ah, of course the problem is that GtkFlowBox doesn't behave as one might expect unless it's in a GtkScrolledWindow: https://gitlab.gnome.org/GNOME/gtk/-/issues/897

A gtk::ScrolledWindow with both horizontal and vertical scrolling disabled seems to have the same issue. It may be possible to address this by subclassing/wrapping gtk::FlowBox with custom size allocation, though I'm not sure it's worthwhile.

Edit: The fact that gtk's maintainers don't seem to consider this worth implementing in FlowBox likely suggests it's not that simple.