Define and use sx-question-mode-display-buffer-function
Make sx--data-here throw error on failure.
Section Comment
Refactor sx-question-list--unread-count into a function.

Less bookeeping
sx-question--mark-read returns nil if nothing changed
Refactor sx-question-mode--window into a function.

It made little sense the way it was, and it was complicated. Now that
it's a function, it's always up to date.
Refactor sx-question-list-display-question into 2 other functions
Maintain good line position while navigating the question list.
Merge branch 'master' into improve-display-engine

Conflicts:
	sx-interaction.el

Conflicting functions:
	sx--data-here
Redo sx--data-here take &optional noerror
Merge pull request #126 from vermiculus/improve-display-engine

Improve display engine