Simplify the lightdm mock to make future greeter improvements easier to test.
I simplified the mock liblightdm to avoid separate files for the Private classes. That can all go into the main files. The separation isn't worth wading through the files to find what you want.
And I dropped the mock LightDM plugin entirely. (opting instead for a tiny "mock()" API call on the real plugin that returns an object that can be used to manipulate our mock liblightdm, if we're in testing mode)