Enable building Qt WebEngine without accessiblity enabled.

This adds guards that for QT_NO_ACCESSIBILITY being not defined
around the code that uses accessibility types.

It disables the quicknanobrowser and quicktestbrowser examples
which need Qt QuickControls, which has a hard dependency to
accessibility being enabled.

Task-number: QTBUG-43305
Change-Id: Ifa39e3ce447ebc2124f52e00b29b1d7d2231035e
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>