overlays: More work on inventory overlay component

A better structure for the component, with more well-defined methods and
some properties as more readable shortcuts for FIFE-based stuff that is
needed every now and then.

Also adds more clear documentation, some TODOs and error handling.

As for `remove()`:
I left some questions about how the base image is supposed to be handled
in the respective code. This should ideally be clarified with FIFE to not
cook up some custom haphazard "solution" - maybe we can even get a nice
interface built into FIFE.