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/app.module.ts

@@ -22,6 +22,7 @@ import {MatProgressSpinnerModule} from '@angular/material/progress-spinner';
 import {MatRadioModule} from '@angular/material/radio';
 import {MatSnackBarModule} from '@angular/material/snack-bar';
 import {MatTooltipModule} from '@angular/material/tooltip';
+import {OverlayModule} from '@angular/cdk/overlay';
 import { YAxisDefinitionComponent } from './y-axis-definition/y-axis-definition.component';
 import { QueryAutocompleteComponent } from './query-autocomplete/query-autocomplete.component';
 import { LimitByComponent } from './limit-by/limit-by.component';
@@ -101,7 +102,8 @@ import { MarkdownModule } from 'ngx-markdown';
     MatTableModule,
     MatTooltipModule,
     BrowserAnimationsModule,
-    HttpClientModule
+    HttpClientModule,
+    OverlayModule
   ],
   providers: [{provide: MAT_DIALOG_DEFAULT_OPTIONS, useValue: {hasBackdrop: true}}],
   bootstrap: [AppComponent]