The HTML and Markdown preview modes both use an OutOfProcessWebView to
render the preview pane, and we were instantiating this view from GML.
This caused us to always spawn a WebContent process alongside every
TextEditor instance.
Fix this by deferring the OOPWV construction until we actually need it.
This makes launching TextEditor on a text file quite a bit faster. :^)
Instead of tracking this stuff ourselves at the application level,
we now just act as an intermediary and pass along the information to
the windowing system.
