Shiny application satellite window minimal infrastructure

This set of changes does the following:

1. Changes the source command to "launch" (this will become run/reload later) for Shiny application files
2. Executes the appropriate `runApp` command when a Shiny application is launched
3. Adds a dedicated satellite window viewer for Shiny applications
4. Adds a function, `rstudio::shinyViewer` that launches the satellite window
5. Sets the option `shiny.browser` to point at `rstudio::shinyViewer` by default. 

An accompanying change to Shiny itself (currently on the `feature/shiny.browser` branch) causes it to use the `shiny.browser` option RStudio sets to launch the app.