…he grid.

…stent file and then updates the the new value. This doesn't work when disabling the overlay, which still requires a viewer update to refresh.

* Simplified the API down to a Viewer/EventViewer 'overlay enabled' function, and retrieving the selected `overlay` for the particular viewer.
* While the whole API can be accessed via CustomOverlay, most will provbably access via the Viewer props (isOverlayEnabled/overlay/userOverlays)

…r more accurate display #2719*

* Currently very broken
* Partially working:
   - Draggable Guides as multiple canvases
* Not working:
   - Overlays
   - Transparency on anything
* Also, some wicked bad lag when initially loading CP (on my machine). Still determining if this is due to code in this branch or CP overall.