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/plot.service.ts

@@ -239,7 +239,7 @@ export class RenderOptions {
   constructor(
     public height: number, 
     public width: number, 
-    public keyOutside: boolean, 
+    public showKey: boolean, 
     public renderLabels: boolean) {}
 }