projector: fix controls help dialog button (#6024)

## Motivation for features / changes

On the embeddings projector, on the top left corner of the scatter plot
there's an help icon (with a question mark) that when clicked should
open a dialog showing an help for the controls in 3D and 2D mode.
Currently the icon doesn't work and if clicked nothing happens.

## Technical description of changes

Set a custom function on the `on-click` event of the
`paper-icon-button`.

## Detailed steps to verify changes work correctly (as executed by you)

- Open the embeddings projector
- Click on the help icon (question mark, tooltip "Help with interaction
control")
- The help dialog should appear at the center of the page

Co-authored-by: Marco Romanelli <mromanelli9@gmail.com>

Author: mromanelli9

tensorboard/plugins/projector/vz_projector/vz-projector.html.ts

@@ -358,7 +358,7 @@ export const template = html`
           ></vz-projector-metadata-card>
           <paper-icon-button
             raised
-            onclick="help3dDialog.open()"
+            on-click="_toggleHelp3dDialog"
             icon="help-outline"
             id="help-3d-icon"
           ></paper-icon-button>

tensorboard/plugins/projector/vz_projector/vz-projector.ts

@@ -704,4 +704,9 @@ class Projector
     }
     this.notifySelectionChanged(state.selectedPoints);
   }
+
+  _toggleHelp3dDialog(event: MouseEvent) {
+    event.preventDefault();
+    (this.$$('#help3dDialog') as any).open();
+  }
 }