[Feature request] Links could be marked if their target does not exist

[Oliver Joos]

This idea came up while discussing links to anchors (see bug 380844).

Links to other notebook pages (those with : and +) or to attachments could e.g. turn red if their target does not exist. For external links like http:// this marking could be inefficient because targets may vanish just because of a weak WLAN signal. Therefore internal and external links could be made different, e.g. valid internal = green, invalid internal = red, external = blue. Or we define clearly when an external link is "invalid", e.g. we check external links whenever a page opens, and mark them invalid if checking failed within the last hour or more. I'd prefer the first, simpler approach(with the 3 colors).