Add return, property, and parameter type hints to code

First round with return type hits where we have a solid API.

Parameter type hints where we have a well defined API, biggest exceptions are the plugin validate() method which has no defined interface and has type hints that conflict with the code (requesting an integer and passing it to a method requesting a string). Also skipped type hints where the parent interfaces or implementations do not allow adding such.

Automatically closed - issue fixed for 2 weeks with no activity.