Add new Task convenience class

Wrapper around GTask, which provides some minor compatibility between C
functions and C++ slots, etc.
RecordingForm: use applications-multimedia icon

Instead of using the 'document-open' icon (which doesn't fit very well), use the
applications-multimedia icon to make it more obvious that the button will open
the file in the media player.
TreeView: clear view columns when model is unset
Add Application::import_file_async()
Refresh view when recordings are imported

Add a signal_database_changed() to application.  In the main window, listen for
the signal, and re-query the database when this happens.
Treeview: clear view columns even when model is not null