Make the test a bit more sane.

Unactivating a window is not really supported, nor has it ever really
been, so activate another window instead. This incidentally
also makes the test work cross platform.

Change-Id: I6e593e9b7972dd5c5038c8d18a42be90bf19248c
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Change error text for missing platform plugin

Model error text after the default Windows error message for missing
.dll's.

Change-Id: I5328a0fe681ff5104dcf41668d9b08d6a6198c08
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>