This is the first step towards implementing support for different
cloud providers.

The use of File objects is replaced with the interface IFile, which
has similar operations. A LocalFile implementation is provided to
deal with local contents in the same way it was being done before.

Some system-wide storage operations are abstracted in the interface
IDocumentProvider. A LocalDocumentsProvider implementation is
provided.

Known issues in this patch:

* Dummy filesystem code has been removed. This should be provided as
  an implementation of IDocumentProvider in case we need it, but that
  will probably not happen.

* Sorting is not implemented in LocalDocumentsProvider yet. It seemed
  not to be working anyway.

* Code to get directory thumbnails has been commented out. This code
  will probably belong to the IDocumentProvider/IFile implementations.
  We might want to have a different approach instead of reading
  hidden thumbnail files, something similar to what the native
  Start Center does.

Change-Id: Ib0882c69a0b24e28d2346bbe0154db01d1102b06