Consolidate includes and lib directories

[GaryJones]

The lib directory only contains zoninator_rest/, so while a previous theory might have been to extract this bundle of code into a package, it seems like it's relatively well coupled to only being used in the Zoninator plugin.

The includes/ directory contains four classes, two of which extend classes from the lib/ directory.

For ease of development and clarity of understanding, it would seem sensible to consolidate these into a src/ directory (along with the widget and Zoninator class files).