The behavior of the cog varies when:
* The user is logged in/out.
* Page Previews is enabled as a beta feature.
Since the behavior of the cog doesn't vary per-preview, it can be
determined at boot time and passed to the renderer. However, in order to
keep the renderer stateless, we pass the behavior to it when a preview
is added to the DOM.
Changes:
* Add the mw.popups.createPreviewBehavior factory function, which
returns an object that encapsulates how a preview responds to the user
dwelling on it, abandoning it, and clicking on the cog.
* Invoke mw.popups.createPreviewBehavior at boot time and pass it to the
mw.popups.changeListeners.render change listener, which then passes it
to mw.popups.Preview#show.
* Make mw.popups.Preview#show responsible for binding event handlers
and configuring the cog based on the behavior.
Bug: T146889
Change-Id: I39d7d0afd7b1fe896019a1b3a82ee907bfb20edd
