Various improvements to favicons

[sissbruecker]

This adds various improvements to how favicons work:

• Changes the default favicon provider to use a higher-resolution image to give better results for high DPI displays
• Adds a {domain} placeholder for custom favicon providers that require only the domain / host name without the URL scheme
• Automatically tries to detect the correct file extension for the loaded favicon
• Adds DuckDuckGo as an alternative favicon provider to the documentation

Closes #465

[AprilNEA]

Good job, my icon service hasn't worked for a long time now.