Intensity Graph: Send cursor location to PV

Provides #470 

May not be the final implementation, since VTable currently has limitations.

Still, it sends the cursor info to a PV, doesn't break existing API, and it's probably better than the existing `XYGraph.addPixelInfoProvider` code. That one is limited to updating the tool-tip based on cursor information. This sends the cursor location to a PV, and this can then be used to update the tool-tip, but also any other property of this widget, other widgets, or other PVs.