list-view: Allow ListView to shrink

We were forcing a constant width for ListView rows in order to limit
the maximium width of the content. This prevented the main Boxes
window from being resized to values smaller than that.

Now we store a max-content-width value in the ListView and whenever
the allocated width for the main window is smaller than that, we
just let the listbox fill the available space.

This is somewhat a responsive behavior for width allocation that
should cover small screen devices or tiling displays.

Hopefully Gtk4 will introduce a max-width CSS property that would
ease the implementation of these use cases. Alternatively, Gtk4
might have a constraint layout machinery that would obsolete this
issue.

Fixes #76
4 jobs from list-max-content-width in 38 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #166088
flatpak

00:34:29

 
  Review
passed #166089
review

00:00:56

manual #166090
allowed to fail manual
stop_review
 
  Cross Distro
passed #166091
fedora:rawhide

00:03:01