Dark mode, Do-Not-Track, general improvements

• Right-click a station to add or remove to favourites directly
• Add settings menu with "Do-Not-Track" option, disables sending station listening events to radio-browser.org
• Add About dialog
• If a station is already in your favourites, you'll see a little star in the title
• Randomly selects one of the available radio-browser.org servers (was always de1 before)
• Favourites are now stored in a local favourites.json file to improve app startup time
• Fix broken dark theme support (elementary and Adwaita dark look fine now)