Exclude accessibility files on windows

This allows the BrowserAccessibilityFactoryQt and
BrowserAccessibilityQt classes to also be used on Windows, and this
also remove our dependency on ATL, which is only easily available
for Visual Studio Professional users.

Change-Id: I1a9c047f0343252a0b6fcc215b84e4c2f3d664b4
Reviewed-by: Andras Becsi <andras.becsi@digia.com>