Open folders directly from the Music app

### Problem

Currently, the only way to select and recursively add all music files contained in a folder is through the file manager, using the "Open In" context menu. The app window offers no equivalent functionality, as it only lets you select one or more files, not folders.

### Proposal

On [Amberol](https://apps.gnome.org/Amberol/) — which is a very similar player — you can press the _Add Folder_ button to select any folder and include all playable audio files contained within into the playlist.

This would require recursively traversing the directory structure to gather all music files inside. Note that I organize my music in an `Artist > Album` structure, and I often want to play all albums from an artist. Amberol lets me select the Artist folder to include everything inside.

### Prior Art (Optional)

_No response_