Add toggle during search to show all categories

[h-hyuuga]

This PR reverts the behavior introduced by #1312, and adds a toggle for searching all categories/current category.

Supercedes #1340

The toggle is only visible when:

• A search is active
• A library is grouped by categories
• Show all categories is disabled

[Jays2Kings]

this isn't what i meant before, I meant adding a button in the search bar

and this icon would only show while the search bar is active (with the tooltip "Show all categories")

I'm not sure how feasible it is right now I can take a look later if it sounds like too much