Add playlist feature in downloader plugin + custom menus in plugin system

[th-ch]

This PR:

• updates the plugin system to support custom menus
• update the downloader plugin to support downloading playlists (creates a folder with the playlist name) - [Feature Request] Playlist Downloader #190
• adds a mutex in FFMpeg conversion (downloader can be parallelized but only one FFMpeg command can run at a time)

[Araxeus]

@th-ch I thought to add another menu item 'Choose Folder' - which lets you open a file dialog to choose download directory
Is it ok if I add it myself to the downloader plugin menu.js, or are you planning to addon adding that yourself?

[th-ch]

Hey @Araxeus, feel free to do it yourself, this contribution is more than welcome!