Enable the declaration of non-executable resources in toolkits

[Updownquark]

The MuisRegistry.xml file should have a resources section where resources can be declared by path or URL and given a name. These can then be referenced by namespace:tagname notation in a MUIS file, for example, from an image tag. Anything that can use resources must have the ability to look the resources up directly from a URL though, as not all toolkit resources should generally be advertised. Do this through a utility that takes a string and returns the resource whether it is a toolkit reference or a URL.

[Updownquark]

Defined ResourceMapping and implemented functionality to pull in tags like

<resources>
    <resource tag="tag.name">path/to/resource/file.ext</resource>
</resources>

and to refer to them from the document, like the src attribute in image