Add makefile for MediaObserver project.
Add initial interface of DirectoryObserver and test.

Test case must be failed.
Provide getting home directory.
Use Gtk in tests for correct initialization.
Get names of files in dirrectory.
Make test for opening home directory as passable.
Merge branch 'enumerate-children-of-dir' into init-media-observer

Conflicts:
	MediaTests/main.cpp
	MediaTests/makefile
Use pkg-config in makefile of MediaObserver.
Add class of abstract observer.

I am planning to use this class for inheriting
for observers which get media files from different
places, e.g.:
  * directory on file system;
  * user's media on some website (e.g. vk.com);
  * user's local playlist base (for better usability);
Inherit DirectoryObserver from Observer.

Add stubs on virtual methods.
Add test on getting media files from directory.
Add utility for checking supported media files.
Enumerate supported media files in observer.
Get mp3 files from DirectoryObserver.
Add call of Update for directory in get_media_files_from_directory.
Add ape media file.
Add Flac media file.
Add Wav media file.
Remove useless comment in Mp3File class.
Update makefile of MediaCore subproject.

Add new media files to build.
Use pkg-config for gstreamer and glibmm.