Fix crash on exit (BL-487)

The main crash got fixed in geckofx, but this change fixes some
missing dispose calls that might also have contributed to the
problem.
It would be helpful in cases like this to make your own comment in the PR, something like "this method moved unchanged to Program.cs".

These two methods actually feel more natural to me where they were, in a file dedicated to the browser. What would you think of leaving them there, making at least SetUpXulRunner public, and adding a comment that the program's startup code should call it (and presumably a parallel routine to shut it down)? Don't worry if it's a lot of trouble, but I'd prefer to keep the bulk of the code related to managing geckofx in the one class.