Preemptively add a potential future file to .gitignore
As per @pjcozzi's request, make `Polygon` and `ExtentPrimitve` asynchronous by default again.

Make `DynamicPolygonVisualizer` set asynchonous to false to fix the original issue.
Merge pull request #1128 from AnalyticalGraphicsInc/revert-1122

Backout #1122
Timeline CSS tweak.
Merge branch 'master' into polylineGeometry
Updates based on review.