Loading data files

[sbusch42]

We need a way to access data files, that works for both the native and the web case.
I'm thinking along these lines:

• A utility struct, which is instantiated in the binary, that somehow knows and returns the base data path. In the native case, this would be a local file path (see cpplocate for how we could implement it), and in the web-case it would be the base URL the website is served from.
• A Loader interface takes this base location + relative path to load a file and return the contents. This will most probably need to be asynchronous, as it needs to be in the web case.
• Do we need further abstractions? E.g., if on WebGL a texture can only be created from an JS Image object, do we have to abstract that or are there possibilities to get the binary data, so we could use the same image loading routine as for native?