[FEATURE] Compilable Assets

This feature consists of:

* An interface for CompilableAsset
* An interface for AssetCompiler
* An abstract CompilableAsset ViewHelper
* An abstract AssetCompiler ViewHelper
* An additional processing step while sorting Assets

The feature works by postponing building of CompilableAssets until every Asset has been sorted. The CompilableAssets are grouped by their relationship to an AssetCompiler which is inserted and configured as an Asset ViewHelper [1] which is used to render all related CompilableAssets.

[1] Note that this means AssetCompilers as well as CompilableAssets can be configured through TypoScript in the same level of detail as any other Asset type.