Allow defining platform specific gstreamers colorconversion-plugins

There's already support for special video conversion elements in src/plugins/multimedia/gstreamer/common/qgstreamervideosink.cpp for i.MX and Nvidia. This should be made more generic to support other conversion plugins, as well (like Qualcomm's qvconv). Conversion plugin selection would be preferred at compile time (alternatively at runtime).

As an additional benefit this should avoid the annoying warnings ("Failed to make element  from factory imxvideoconvert_g2d" / "Failed to make element  from factory nvvidconv") on other platforms (which could be considered a bug in itself).