make the legend movable

The legend ("key" in Gnuplot speak) is no longer part of the image.
Instead it is a floating&movable overlay.

In the gallery we still use the legend/key in the image.

pdb-js/src/app/visualization-page/visualization-page.component.ts

@@ -243,7 +243,7 @@ toggleGallery(event: Event){
     const config = this.createPlotConfig();
 
     const renderOptions : RenderOptionsMap = {
-      'main': new RenderOptions(results!.offsetHeight-1, results!.offsetWidth-1, false, true),
+      'main': new RenderOptions(results!.offsetHeight-1, results!.offsetWidth-1, true, true),
       'thumbnail': new RenderOptions(200, 300, false, false),
     };