Make 'Home' button useful

[mattgibbs]

PyDMMainWindow's navigation bar has a 'home' button on it - this thing is meant to always take you to a file that makes sense at your site. For LCLS, for instance, this would be LCLSHome. There's even a home_file variable on PyDMMainWindow, but it is completely unused.

We should:

• Make a command line argument (--homefile?) and an environment variable (PYDM_HOME_FILE?) that can be used to specify a path to a display to use as the home file.
• Use that to set PyDMMainWindow.home_file. Then, use the home_file property to navigate back to the home screen when PyDMMainWindow.home() is called.

[mattgibbs]

I should say: I wrote most of the MainWindow code, so the fact that the 'home' button is not useful is all my fault!