Refresh category page UI
See screenshots of the new appearance below. It implements the new design.
I think the only thing worth pointing out is that this changes the Add-ons category so it’s no longer a special case, which I think makes it less usable. Previously it presented codecs/fonts/etc. in separate tabs; now they’re all listed together like in any other category.
I don’t think the Add-ons category is worth further development time at the moment, though.
Helps: #1111 (closed)