Evaluate codebase for use of 'reference code snippets'

[elbenfreund]

Because gnome-shell-extensions documentation is so lacking, it is common advise to copy and paste from its provided reference extensions. Most prominently the example extension to see how basic functionality can be implemented. In order to improve documentation of hamster-extension specific changes to that code we should do:

1. Mark all such instances of c/p explicitly with reference to the origin
2. Provide elaborate comments along the code that explains where we deviate from the reference implementation