* [UWP] Initial support for AccessKey
Fixes#1672 by adding AccessKey, AccessKeyPlacement and horizontal
and vertical offset properties to Windows specific VisualElement class.
Also handles AccessKeys for tabbed pages (ContentPage).
* Fixing support for updating AccessKey on tabs
Refs #1672 and fixes issue with AccessKey not being updated
after tabbed page and its children had been created. The problem
was that the Reset event in OnPagesChange wasn't handled correctly.
We now add listeners to the child pages of the tabbed page which
makes sure AccessKey gets updated if user makes changes to tab object.
* Moved shared code to internal class AccessKeyHelper.
Updated foreach-loop and replaced spaces with tabs.
Henric Müller