Add a "Scan All" to the Movie and Series Sections

Add a `Scan All` button to the Movies and TV series section so that I don't have to go through each Movie and Series one by one and hit `Scan Disk`.

Note that the tasks `Update all Episode Subtitles from disk` and `Update all Movie Subtitles from disk` do not actually update the files from disk, but the button `Scan Disk` does. I'd like a feature that works by doing `Scan Disk` for my whole library.