Add some kind of dependency-management for js&css sources used by the PageComposer

Currently there is no dependency management at all on the resources used by the pageComposer. Since we can't use Maven for this (unless we publish are own artifacts containing jquery+ext+plugins for both) I've started work on a very simple DependencyModel implemented in Java.

Together with this, I will restructure the FS structure of these dependencies, which should allow for easier updating/testing of different versions of an external library like EXT or Jquery.