Show .hidden folders in Library and Files tabs

My entire music library is in one folder. Most of the first-level subfolders are albums, but some of them are special folders that hold certain types of music. I make these special folders stay alphabetically at the top of the folder listing by giving them a .dot name. This turns them into "hidden" folders, although I don't treat them as such, and always have all hidden folders shown in my file manager (Thunar).

However, clem treats them as hidden folders, and ignores them when building the music library and when displaying folder contents in the Files tab. AFAIK, there's no way to toggle this in the GUI. The first two commits in this PR address my problem by displaying all hidden folders by default, although I don't know enough to make this toggleable in the GUI.

The third and final commit leaves all files visible in the Files tab, not just the ones that match the music file filter (*.mp3, *.ogg, etc.). Those that don't match the filter are left visible, but disabled. I like this, because I have a lot of other non music files that I'd like to see when browsing via clem's Files tab, though of course I can't do anything with them from within clem. I guess the question is, what is the Files tab for? For me, it's a built-in mini file browser with which I can browse anything, and also queue music.

Thoughts?