This uses exactly the same code, with one exception. In the previewer
`self` became `self.parent` in order to have action on the
browser. And in the browser, some `self` become `self.previewer` to
access the previewer. (Some function having an action on the previewer
starting from the browser now are separated in two. One version in the
previewer doing the same thing. One version in the browser, calling
the version in the previewer if it exists.)
Preview dialog now takes a QWidget in general, not necesarrily a
Browser. The parameter is called parent
