Refactor _htmlPane usage.

This makes it possible to display a Conversation in a standalone _window_ (not
tab), as long as there is one mail:3pane window available somewhere.

This is currently not leveraged anywhere.
Partially honor the "open new messages in..." pref

- This applies to both the "pop-out" button in the conversation toolbar and the
double-click-in-thread-pane feature.
- This comes with limitations (no tooltip, no context menu, doesn't work if the
3pane-view is closed).
And make middle-click _always_ open the conversation in a new tab
Merge branch 'master' into html-reply