This is useful, for example, when we want to test the virtual
keyboard on OS X. It uses the usual QT_IM_MODULE code path
provided by QPlatformInputContextFactory if that environment
variable is set. If not, it defaults to QCocoaInputContext.

Change-Id: Ib917d7bd2f4e86eacbc50dd7592cc288a9a702de
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>