Fixup camera interaction relayout event data

[etpinard]

Another bug I found when working on gl3d annotations.

On scene drag/pan, we fire a plotly_relayout event mocking the equivalent Plotly.relayout call that would generate the current view.

Previously, the mocked event data looked:

{ scene: { eye: {/**/}, up: {/**/}, center: {/**/} }

(or scene2: // ... for multiple-scene graphs) which isn't compatible with our plot schema

Now with this PR, the mocked event data is:

{ 'scene.camera': { eye: {/**/}, up: {/**/}, center: {/**/} }

cc @monfera

[alexcjohnson]

nice catch! 💃