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.

Backport of d95b9295

 from 5.5

Change-Id: I2a2a0a6196fcb3baa39603d9c929183b454f39ed
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>