Add option to run a window manager

Allow run a window manager for UI tests running using virtual display
feature. If the window manager command specified in the properties file
is found then it will be run before open any browser window. This will
allow the browser window be maximized.
Assuming someone does not install the deps from our optional requirements, the `import` statements would cause the tests to die prematuraly. Should we do a ``try/except ImportErrror`` here?