Support for R Markdown documents in the Viewer pane

This change adds support for showing R Markdown documents in the Viewer pane.

- A new menu appears in the editor toolbar for HTML-based R Markdown documents. It's similar to the menu shown for Shiny applications and shows the value of a new UI preference that indicates the preferred viewer (pane or window).
- When the preference is set to Pane, HTML documents that finish rendering are shown in the Viewer pane. 
- When the preference is set to Window (the default), the existing codepath runs more or less unmodified.

The code specific to the satellite window has been factored into `RmdOutputFrameSatellite` and put behind an interface (`RmdOutputFrame`), and a new implementation of the interface for the pane has been added (`RmdOutputFramePane`).