Polling of available printers in now asynchronous (#264,#256)

- Increased polling interval from 1s to 10s
- Use QtConcurrent::run to initiate a poll asynchronously to main event loop
- Do not initiate a poll if the previous poll has not completed
- Use copies instead of references to mCurrentAvailablePrinters in public API

glabels/PrintView.h

@@ -57,7 +57,7 @@ namespace glabels
 		// Slots
 		/////////////////////////////////
 	private slots:
-		void onAvailablePrintersChanged( const QStringList& printers );
+		void onAvailablePrintersChanged( QStringList printers );
 		void onModelChanged();
 		void updateView();
 		void onFormChanged();