"Add a folder" shortcut

Currently new folders can be added by:

- [it will happen automatically if "Listen for file changes..." is enabled]
- "Rescan for new and changed music files"
- "Clear and rescan"
- If it's a separate folder to the existing music library, adding it as a new folder in the settings page.

If you don't want listening enabled, the trouble with the second two is the length of time they take for a large library.

It would be nice to directly add a folder of music files.