More compact handling of the filter bar on small screens

[helen]

The "filter bar" takes up a lot of vertical space on narrow screens and ends up becoming a point of focus when it is more typically secondary navigation. It's used for theme install, plugin install, and both views of the media library. For themes, it is especially bad because the search input separates the feature filter toggle from the filter controls themselves.

Screenshot:

[helen]

I've manually added a keyword of "needs-approach" to this as an experiment, and that's exactly where this is. It needs design approach (both UI and UX) as well as testing.

A thought from me just to get us jump started, no mockups made: use icons up by the h2 (maybe also collapsing down the upload link to an icon?), with a dedicated one for search instead of one big thing. The settings Dashicon would work quite well for filtering, I think. This carries the caveat of icons not always having universal meaning, so it would definitely require testing if we even felt it was a good idea.

[afercia]

Related: #38086