Make the progressButton use signals instead of functions

Although the naming is still a bit off, it's much cleaner to use signals instead of functions. It's
also more in line with how default QML components handle these kind of situations

CURA-6006

plugins/Toolbox/resources/qml/Toolbox.qml

@@ -90,7 +90,7 @@ Window
                 licenseDialog.show();
             }
         }
-        
+
         ToolboxLicenseDialog
         {
             id: licenseDialog