Support setting webview background color

[russkyc]

This should allow for setting the background color of the webview using:

// Set cef background color
WebView.Settings.BackgroundColor = Color.Black; 

This should be useful for:

• Dark themed apps, so that the webview does not flash white while loading content
• Consistency, we will be able to set a background consistent with the content that will be loaded

[cssecautomation]

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