Missing Document.execCommand

[vegegoku]

Elemental2 version : 1.0.0-RC1
The document object provided by DomGloabl does not have the execCommand function property
as a workaround i extended the HTMLDocument type and added the function and it worked out

@jstype(isNative = true, namespace = JsPackage.GLOBAL)
public class MyDocument extends HTMLDocument {
public native boolean execCommand(String aCommandName, boolean aShowDefaultUI, String aValueArgument);
}

the function seems to exist for gecko_dom extern in closure compiler
https://github.com/google/closure-compiler/blob/41a4dcce5ec80df173e3b5f22f93eabb6ee525a3/externs/browser/gecko_dom.js#L497

other links
https://www.w3.org/TR/clipboard-apis/

https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand

[jDramaix]

I'll cleanup the extern files very soon and move standardized api to the related w3c_xyz.js files

[jDramaix]

This will be fixed when #86 is fixed