Add the IsEnabled method used by the Tools to the Custom Visualizations

As a Spotfire Developer the customer would like to have an enable/disable option for custom visualization the same way as we have IsEnabled() method in the tools today(CustomTool).

End-goal:
If this is implemented the developers can perform specific checks on the loaded Document to see if all the requirement of the visualization is met and therefore can be added. In this case the developer need specific types of data that must exist beforehand, if not the visualization will not be rendered correctly.

This is probably the same in many applications where certain visualizations have specific requirements on the data before they can be loaded correctly.