Rescan library in thread.
Use mutexs and timeout to make thread safe.
This may allow add the progress indicator in the main window. Allowing keep listening to music while analyzing the library
First attempt to update the library correctly.
 * Now only clean remove files. Before pragha not do it.
Rename some functions.
Add a label to show info when scan/update library
Upgrade the library correctly.
1. Clean removed folders in preferences.
2. Clean removed files checking the entire database.
3. Update files changed based on st_mtime (time of last modification) stat().
4. Add new folders added in preferences.
Fix typo.
Clean a bit the dialog.
Remove the spinner.
Port partially last_rescan_time to PraghaPreferences
Divide and conquer?. Clean folder estructure view a bit. and remove f*cking assert!!.
Divide and conquer 2:. Clean library view order bit tags.
Use db_begin/commit_transaction() when scan library.
Remove the lists lib_delete and lib_add, and use library_scanned.
Port partially library_dir and library_scanned to PraghaPreferences.
Port the rest to global instance of PraghaPrefrences. So, forget of cwin.
Fix seg fault on  pragha_preferences_get_filename_list().
TODO: Reapend incompatible db upgrade test.
D'Oh!  D'Oh!  D'Oh!.
Library_scanned list is saved only when finished the scanner!.
Set title of dialog according to task.
Add pragha-scanner.c to POTFILE
Add a mutex to ensure no concurrent writes.
Revert "Add a mutex to ensure no concurrent writes."

This reverts commit 0f01d3433c587efacf1cd2a8e10a07983380c6a9.
When rescan the library only get a list of all track scanned in a thread. Olny when finish it update the database in the main thread.
TODO: The update procress can be more complicated..