Add capability to capture/make screenshot content of a Panel and save it to the file

[ichan-mb]

This add one ScriptPromise called capture to the Panel.

Example

<JavaScript>
    module.exports = {
        save: function (args) {
            p.capture().then((file) => {
                console.log(file);
            })
        }
    }
</JavaScript>
<StackPanel ux:Name="p">
    <Text Value="Demo capture" />
    <Button Text="Save" Clicked="{save}" />
</Panel>

This PR contains:

• [ ] Changelog
• [ ] Documentation
• [ ] Tests