Add grid settings to preferences (#224)

- Allow origin to be either at the top-left corner or center of label (#174)
- For non-rectangular labels (cd, round, etc.) the origin will still
  always be at the center of the label.
- User controllable grid spacing

glabels/PreferencesDialog.h

@@ -47,6 +47,10 @@ namespace glabels
 		/////////////////////////////////
 	private slots:
 		void onUnitsRadiosChanged();
+		void onGridOriginRadiosChanged();
+		void onGridSpacingSpinChanged();
+		void onGridSpacingResetButtonClicked();
+		void onSettingsChanged();
 
 	};