Should support previewing PNG and other image resources

[BretJohnson]

Currently in the resource picker UI proppy just supports preview for colors and strings (from what I can tell).

It would be good to support PNG files and other image resources here, as we do this in the current Android property editor. See this comment from @garuma on https://github.com/xamarin/designer/pull/648:

We should at least support plain bitmaps, the value of those is the full path to the .png file.

For the other drawable type we query the rendering server for a image preview of those (shape drawables, nine-patch, ...) in the Xwt version so ideally that's a case that proppy should support via an extension point.