Allow widgets to determine when they'd like to be added to the dashboard

[jovanni-hernandez]

... sometimes the widget may have to retrieve a large batch of data before a visualization is ready. Currently, the widget would just attach to the dashboard and the loading indicator would continue running. Ideally, the widget would be able to indicate to the dashboard when it would like to be added, that way loading can be done transparently, with the widget being added when it is completed.

Previous attempts at implementing:

• I tried loading widgets into an invisible div and moving it to the dashboard once it was loaded, but that led to the widget being remounted each time.
• I tried just make the widget invisible while attached to the dashboard, but this just creates a blank widget on the dashboard

Future: It looks like react-grid-layout may have to be modified to support this feature.