Rename generic plugin names

We should not use names as generic as "plugin.dll" and "private.dll".
This will cause a lot of confusion when deploying.

Task-number: QTBUG-31348
Change-Id: Iee18e4d0cd713b63ff1b99dc0db31123b90666ec
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>