Removing files, more optimalizations and closing on first release

• Library now checks for missing files in selected paths and if library contains a file that is not present, it removes it from library.
• Maintenance thread was significantly slowed down to work every 10s, but on the other hand can be now invoked while waiting - this is used by inotify for immediate file detection and import.
• Imports are not disabling the album view until the model is being filled - if you are importing more files, you can play already imported files while new ones are being imported. (this also fixes the scroll back to top of the icon view on every refresh/import)