Got rid of the ScrollView in MessageView

The ScrollView around the WebView caused all sorts of problems. This
change removes the ScrollView and uses the undocumented method
WebView.setEmbeddedTitleBar() to set the MessageHeader view as "title
bar" of the WebView. This allows MessageHeader to scroll away making
more room for the WebView.
All of the "magic title bar" code was originally implemented by Jesse
for Kaiten.

Because WebView doesn't support a scrolling footer we can no longer
support scrolling buttons or attachments at the end of the message. Now
users can switch from message view to attachment view via a button just
below the message headers.

I also copied some code for which I was too lazy to create a separate
commit. It allows to display attachments we didn't use to show by
clicking on a "More..." button in the attachment view. Those
attachments are mostly images referenced by the HTML part (e.g.
background images).

Fixes issue 3291