Add indicator that search is active

[Raphael0]

Part of a list of smaller enhancements, which I think could help polish the app:

Add indicator that search is currently active

Right now there is no indicator that search is active (beyond there being text in the search bar).

One can easily forget that a search is active which can be confusing when some tracks suddenly happen to be missing. This happens especially in track view when the search string is short (most tracks still appear) and when the user moves to another view and back with search still active.

One solution would be to add a moderately thick border in the accent colour around the search bar.

[martpie]

interesting point, that kind of make sense indeed, I'll think about a nice way to do it. I am working on a bunch of accessibility improvements right now, so that could fall into this bucket.

[martpie]

Fixed in #939