Code reorganization.

* Add pragha_* prefix for most of the files
* Trying to use individual headers for each of the files.
* Rename a few functions, and move some code.

TODO: 
*  Add all code for creating the main interface on pragha-window.c
* Split pragha-playlists-mgmt.c with the code  that manages files and the database.
* Clean pref.c, tags.c, widgets.c
* Port pragha-dbus-c to GDBus.