Theia: Support VS Code Standard Icons in Contributions

[akosyakov]

It is reproducible with https://github.com/microsoft/vscode-java-test:

URLs like hostedPlugin/vscjava_vscode_java_test/%24(refresh). It seems to be that it is not a path, but rather codicons.

[tsmaeder]

The well-know icons are described in the documentation: https://code.visualstudio.com/api/references/icons-in-labels

I've taken the liberty of updating the title; I think it better describes the issue.

[akosyakov]

@tsmaeder Are you fine if I look at it or you were planning?

[tsmaeder]

Please do, @akosyakov