Add confidentiality property to WebSource

[charphi]

Some source might be processed differently if condidential.
For example:

• it should not try to get a favicon because this could leak URLs to external services.
• the cache could be disabled for very confidential data
• the UI could give a visual clue about the confidentiality
• the UI could prevent/warn some operations

[charphi]

Example:

enum Confidentiality { 
  PUBLIC,       // none, gray 
  UNRESTRICTED, // low, blue
  RESTRICTED,   // medium, yellow
  CONFIDENTIAL, // high, orange
  SECRET        // very high, red
}

[charphi]

Done in 1abd182