[meta][mini-browser] Improve the mini-browser

[jankeromnes]

I would like to make small improvements to the mini-browser:

• Make the background white again, which is what every web app expects ([mini-browser] Allow setting background to white #3969, fixed in [mini-browser] Make the mini-browser background white again #4096)
• Fix the live-preview flickering problem that motivated the dark background
• Make the loading animation prettier (it's quite pixellated on retina screens)
• Hide the loading screen when the iframe has errors (e.g. CSP, content encoding, etc, see Web preview overlay hides iframe errors gitpod-io/gitpod#260)
• Make the URL bar editable again (it's quite confusing that it's not, even though you can open any URL in the Preview)
• Fix any weird issues like the Back, Forward and Refresh buttons not working right; the address bar reverting to about:blank; etc.

[jankeromnes]

See #4096 (comment) for context about the read-only URL bar & live-preview flickering problem.

[jankeromnes]

Note, this is what the loading animation looks like on retina screens:

Maybe we could find a prettier / more satisfying animation?

[vince-fugnitto]

Note, this is what the loading animation looks like on retina screens:

Maybe we could find a prettier / more satisfying animation?

I think we should update the default loading animation as well (when launching the application)
It uses the same loading animation and is quite pixelated for sure.

I think we should open an issue for this separate issue so it's easier to track :)