fix(build): Workaround dysfunctional xulrunner stub on Mac OS X (refs #5069)

Bundle all XUL.framework's content into `MacOS/` subdir and provide a
`MacOS/xulrunner` wrapper script as the main CFBundleExecutable.
Merge pull request #72 from eguaj/2.0-A5069

fix(build): Workaround dysfunctional xulrunner stub on Mac OS X (closes #5069)