CategoryView: cleanup

[danirabbit]

Review with "hide whitespace"

• Bump copyright header
• Inline namespace
• Set padding/margin with CSS
• Float searchbar
• Fix scrollbar location
• Reduce number of clamps, scrolled boxes, and stacks
• Make objects way more private
• Fix issues with search and keyboard nav
• Code style
• Remove an idle that caused plugs to load in late and make the view jump on launch every time

[jeremypw]

Code looks good and works as in main.

There is a regression compared to master in that pressing the Up key does not return to the search entry but that was not caused by this PR.

[danirabbit]

Yeah I noticed it looks like maybe a change in behavior in GTK? the flowboxes don't seem to be able to keyboard nav between them either

[jeremypw]

Might be able to fix by putting the EventControllerKey in the CAPTURE phase and catching the Up key before it is swallowed by the FlowBox but its not a big deal.