Rework PraghaScanner:
* Introduce PraghaScannerWorker: A generic object that integrates PraghaScanner and is responsible for getting the songs and add them to their corresponding Container
* Intruduce PraghaScannerFolder, example of PraghaScannerWorker, only scan (Not update) the library.
* PraghaScanner introduce a API to add diferents Workers, (operating in different threads), show progress, cancel it, and get the results, adding it to the database.

FIXME: Taglib is definitely not thread safe. It forced me to add a global mutex. Would have to create an object that implements this most consistently..

TODO:
* Add pragha_databse_forget_container()
* Implemente update the library as a subobjet of PraghaScannerWorker.
* Etc.. .