Better support interactive VisualizationPage - keep PlotlyManager

[victorterpstra]

When using interactive components on a VisualizationPage, the callback will need to get access to the PlotlyManager. We need a mechanism to retain the original PlotlyManager, so we don't have to re-generate.

Work-around:

• In the get_layout_children' store the pm:self.pm=pm`
• In the callbacks, you can now get it back: pm = self.pm

Define a VisualizationPage.get_plotly_manager() -> Optional[PlotlyManager] And store the pm in the VisualizationPage when the pm gets created in VisualizationPage.get_layout

[victorterpstra]

In v0.1.1.1b, added self.pm to VisualizationPage.get_layout. So later we can get it back using self.pm.