[KML Support]: `gx:CascadingStyle` support

When I import a kml file into Cesium that I created with Google Earth, it doesn't display shapes correctly. I would expect the line colors, fill colors, and transparencies to display very closely to what I see on Google Earth. Instead, what I get is a shape with no colors or transparencies. Just a plain white shape. If I create the same kml file with Google Earth Enterprise and load it into Cesium, it displays correctly.

I've traced down the issue to what I believe is a `gx:CascadingStyle>` that is causing the problem. This may not be the whole issue, but if I take the line:
```

```
take the id and move it to the `<Style>` tag and then remove the `<gx:CascadingStyle ` tag, so now it looks like:
```
<Style id="__managed_style_2E63942E081DFE53558D">
```
Now everything works correctly.

My guess is Cesium doesn't parse the `<gx:CascadingStyle>` correctly or maybe it's a matter of not having the style id on the `<Style>` tag. I'm not sure.

Sandcastle example:
[Non-working Sandcastle](https://tinyurl.com/zdb3s58v)
[Working Sandcastle](https://tinyurl.com/22hbr2c)

Browser:
Chrome

Operating System:
Windows 10

Start with the non-working and working Sandcastle examples linked in the issue, comparing their KML input and rendered results. Trace how the Cesium KML importer handles gx:CascadingStyle and Style IDs, then verify that line colors, fill colors, and transparencies render like the working example and Google Earth.

Written by the indexing model from the issue text.