Email addresses and URIs are now clickable when viewing a message in
plain text mode. Clicking an email address moves to the Send tab, while
clicking an URI has the same result as clicking an URI in html mode, it
will ask for confirmation before opening it in external handler.
HTML parser wasn't correctly handling img tags.
Now it also by defaults disabled external schemas to prevent
deanonymisation (even though the renderer actually doesn't support
external schemas at the moment)
Addresses #178
Message will render as user is scrolling down. This prevents interface
freezes on long messages (such as inline img in text mode). Fixes
Bitmessage##366
Also a minor fix in text mode rendering.
