Implements latest Settings spec

[keianhzo]

Fixes #1372 #962 #1440 This PR implements the latests Settings spec which also includes the new user agent switch button in the navigation bar.

[philip-lamb]

Awesome. Will give it a test tomorrow.

[MortimerGoro]

• I have tried to switch between desktop and VR UAs. It works on the current page, but when browsing to other pages I got a desktop viewport while the VR icon was visible in the URL bar

• I think the VR text icon can be confusing for users. Can't we use a similar icon to the one used in Oculus browser? Toggle between the desktop icon with or without the checkmark.

• The VR icon is too close to the URL bar end on the content feed and looks misaligned. Should we hide it on the content feed? (in that case the bookmark icon is not visible)

[MortimerGoro]

cc @jvonitter @thenadj related to the checkmark idea I mentioned:

Firefox desktop uses this icon to enable Mobile/Responsive layout. It changes color when enabled/disabled.

Firefox for iOS uses this icon to go back to Mobile/Responsive layout once desktop is enabled. It shows/hides the blue check when enabled/disabled

[bluemarvin]

I'm not sure if it is related but the first time I ran this PR only the main window was visible, no other UI was visible. I had to restart to see the rest of the UI like navigation bar and tray.

[bluemarvin]

Also, I really do not like the VR in the navigation bar. It looks very heavy and it's purpose is not immediately obvious.

[jvonitter]

@keianhzo new icons!
UA_Icons.zip