Reintroduction of custom maps feature

Reintroduces the custom image layers feature working with OpenLayers. Marked as experimental.

This is my work from about two years ago. Rebased to work with current master. I have no real need for this feature anymore but hope it proves useful for others. There should be high potential to introduce loads of new features based on this.

TODOs:
* Defining multiple layers on a "Layer:" page will allow switching between those layers in a map using the layers of that page. Though, when switching layers the image won't load. This must be an issue of some newer OpenLayers version and must have something to do with how the layers and their measurements are defined in JavaScript since it has been working with old versions of OpenLayers.
* Semantics: "Layers:" isn't an ideal namespace choice for layer definition pages anymore since multiple layers can be defined on such a page.
* Parser hook parameter validation for layer definitions has originally been more feature rich but had to be dropped when rebasing the code for the Validator extensions rewrite. See TODOs in related parameter definitions.
* Documentation and examples.
* Tests!