feat: improve folder support

[huynhsontung]

This PR implements

• Perform recursive folder search when dropping a mix of files and folders into the app. Closes Add Drag-and-Drop Support for Folders #482
• Add an option to automatically add all media files in a folder into the play queue upon opening a single file. Close Automatically add All Videos in the Directory to the Playlist #497 automatically continue to the next song in the folder #441