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
Zooming in message body view / compose works in single steps
irrespective of wheel sensitivity, and info about zoom level is
displayed in percent rather than font pixel size.
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.