Add empty makefile for Interlayer project.
Add tree model for MediaObserver widget.
Fix issues in makefile of InterfaceGtk.

 * add tree model of MediaObserver for building;
 * make correct using pkg-config flag for gstreamer.
Use tree model for MediaObserver instance.
Add list of observers which can be created.
Init class for iterating in obtained items for UI.
Add interface of observer client.
Init mediator for observers.
Add targets to makefile of Interlayer.
Set up main makefile according to new interlayer target.
Move Observers to separate directory.
Add base ObserverState class.
Add state class for DirectoryObserver.
Fix typo in name of method getDirectory().
Add ObserverCreator class.
Add test on creation of DirectoryObserver from state.
Extend test with creation of observer from state.
Add comments to state classes.

Also move ObserverState::ctor to protected section.
Add documentation to observer creator and types enum.
Enumerate type of possible icon in file manager view.