Fix bogus videonode plugin handling

Each plugin must provide its own unique key. Otherwise we will only ever
see one single plugin.

Right now running on i.MX6 is often broken because the imx6 videonode plugin is
not picked up since only the egl one is seen by the system. With the fix both plugins
provide their own unique key so both become visible.

Additionally, introduce a QT_VIDEONODE environment variable. This is useful to specify
which plugin to use. This is necessary in case multiple custom videonode plugins support
the same formats.

Change-Id: Iaa1988f8436dcb938cb9a95e2e0d68a4e92e113c
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>