[FEATURE] Assets with dependency handling and fully mutable configuration

This adds the following:

* A base class for Assets which contains all the logic needed to make an Asset inclusion and build an Asset (if that Asset requires compilation for example)
* Multi-dependency support on every type of Asset (i.e.: a JS asset can depend on two other JS assets and even other types of assets such as web font inclusions)
* A `v:asset` ViewHelper which can add raw Asset bodies to the header
* A `v:asset.style` ViewHelper which can add "style"-family Assets (under a common group, supporting merging as temp file)
* A `v:asset.script` ViewHelper which adds "script"-family Assets again with grouping support and merging into temp files.

Inspect the Asset and AbstractAsset ViewHelper classes for more documentation.

@dreipunktnull would you mind taking a look? This should also work on 6.0 but I've not yet begun the heavy testing there...