KML ScreenOverlay Support

[krupkad]

Related to / part of #873 - breaking this out since I'm actively working on this (see the screen-overlay branch).

CC @hpinkos @mramato and @IanLilleyT from preliminary discussion of this.

Seems like the easiest way to do this is to take a screenOverlayContainer in the KMLDataSource options and put images in there if it is provided, and no-op if it is not. Unsure if this handles "dynamic" overlays, which may or may not be a thing.

[mramato]

Here is the spec for KML ScreenOverlay: https://developers.google.com/kml/documentation/kmlreference#screenoverlay

At first glance, it does look like it's possible to have a screen overlay that turns on/off or otherwise changes based on the animation time. This doesn't necessarily need to be in a MVP implementation but something we should be aware of.