[Release] Australium

[Lootyhoof]

Australium has been updated to v5.1.0.

What's new?
- show a shadow on the tab bar when tabs have one
- improve integration with macOS
- support other GTK-based operating systems
- provide platform-specific custom statusbar styling to ensure it works in all platforms
- Windows/Linux: improve styling of fullscreen interface
- Linux: add private browsing indicator
- Linux: improve appmenu margins with nearest tab
- Interlink: remove sidebar grippy icon to be like Australis
- Interlink: remove out-of-place Windows-specific toolbar styling
- Interlink: add profile manager styling