Add an option to pull all music files recursively into transcoding

I wanted to convert my entire library from 320kbps mp3s to smaller Opus files. It can be quite tedious to either reorganize the entire library into a flat folder so it can be added via add or manually add the contents of every albums folder on its own, so I implemented an Import button in the transcode dialog that can take a root folder and pull all music files from it and its subdirectories.

Tested it on my library and it worked fine. Could use some additional confirmations on it working, of course.

Only issue I found were that you can have name conflicts (which is just a general case issue with transcoding multiple folders to one destination at once). A future feature that might be useful is to preserve the hierarchical layout of songs in the destination.
Use `QVector` instead