Make element index filtered state less subtle

[thupsi]

Right now it's easy to miss the active state of the filter button. Especially if the element index filters become more persistent as suggested in #12590 (comment) this could pose a problem.

I think an easy win would be to make the filter button's active state more conspicuous, maybe by using the "gradient" look that is used when copying entries?

A more significant upgrade would be to display a "pill group" of applied filters, right bellow the search bar, each pill with an"x" icon, for quickly removing filters.