Update preview when word counter is on

Even if preview is not visible, we may need to update it if some
other parts of the program needs it. a new private -needsHtml
property is introduced so that future functionalities can add
hooks more easily.

Fix #151.
Disable word count when auto-rendering is off

The word counter relies on DOM parsing from the preview pane, so
it won't work if HTML is not updating on the fly. This patch turns
the word count pref and its UI toggle explicitly when auto-rendering
is off to make it clear to the user.