Refactored to use minimal Plugin interface

It turns out, the way it was before, it was obfuscating the File and Edit dropdowns in the main window (you could not click on them without first clicking on one of the other dropdowns).  I pared it down to only use what is absolutely necessary, which includes not creating a widget of any kind.  This recipe will form the foundation for `SpyderActionPlugin`, which I will try and get put into version 2.3.