added "new map" button

fixed minor model/logic issues

mapview/3D/MapView3D.cpp

@@ -185,12 +185,21 @@ void MapView3D::showFloorplan() {
 }
 
 void MapView3D::showGrid() {
+
+	// delete the previous grid (if any)
 	if (gridModel) {delete gridModel; gridModel = nullptr;}
+
+	// build a new model
 	GridModel* gm = new GridModel();
 	Floorplan::IndoorMap* im = getModel()->getMap();
 	gm->rebuild(im);
+
+	// remember
 	this->gridModel = gm;
+
+	// update UI
 	update();
+
 }
 
 void MapView3D::draw() {