Improve search performance in FileBrowser

[sakertooth]

Improves the search performance of the file browser by delegating the search to a worker thread. The main thread then builds the tree and displays it to the user.

Fixes #6874.

[Spekular]

Clicking on a file that can be previewed will play the file in its entirely and not require to hold the mouse.

FYI this was already an option via #5787, see also #5427. I'd very much like to keep the option to stop samples on mouse release.

[sakertooth]

FYI this was already an option via #5787, see also #5427. I'd very much like to keep the option to stop samples on mouse release.

Thanks for the heads up. Seems like I overrode that PR a bit. I'll fix it and push the commits soon.

[sakertooth]

Realized that this PR was far too invasive for little benefit other than search performance, so I put the focus on just that rather than a lot of unnecessary changes.

Edit: I was originally worried that trying to build the search tree on the main thread will freeze it up. It does for large enough directories (and very basic queries), but LMMS having to search through the entire root and home directories is largely unnecessary. This PR gives us a much needed boost in performance in general, but the next step would still be to minimize the search space.