Rewrite for Tree View icon

[zenof]

Hi,

Currently the image icon file will be rewrite URL prefix from file:// to /resource thanks to 414eb70

But the Tree View icon won't be rewrite because it use css background-image to load the icon and will unable to load the icon.

If the tree view icon can also be rewrite that will be great for the compatible with extensions that use tree view.

Links:
https://code.visualstudio.com/api/extension-guides/tree-view
https://github.com/microsoft/vscode-extension-samples/tree/master/tree-view-sample