Event handlers missing

[teoli2003]

Gecko implemented Document.onselectionchange and GlobalEventHandlers.onselectstart. [1]
Edge does it too (but both on Document [2])
Blink does it too (on both Document too [3])

These event handlers properties should be added to the spec.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=571294
[2] https://dev.modern.ie/platform/documentation/apireference/interfaces/document/
[3] https://chromium.googlesource.com/chromium/blink/+/master/Source/core/dom/Document.idl

[ehsan]

See #53.

[rniwa]

This is really a duplicate of the issue #54

Fixed this in 6480269.