Support Outlook 2016 signature editor (#7254)
* MS Office 2016 now has a native UI Automation implementation for _WwN controls (Outlook signature editor, spell check field etc), so use this if available. However, move previous _WwN support specifically to IAccessible, as when it is at the Window level it would conflict with UI Automation.
This change is most important for supporting the Outlook 2016 Signature editor.
* Abstract rules for when navigable text should be presented in speech and braille into a _isNavigableText property on NVDAObjects. This property is currently private as the name could change.
The rules were taken from Braille, but the checking of no readonly state for documents has been removed as this was only to protect against browse mode, which is already handled higher up in Braille.
