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.
Create wav, ape and flac media from DirectoryObserver.
Add test on getting sub DirectoryObservers.
Provide getting subobservers of DirectoryObserver.
Prevent getting substr from invalid file name.
Recognize regular file from directory.
Add documentation for Observer class.
Add documentation to DirectoryObserver class.