Update motioncells to inherit from GstOpencvVideoFilter instead of from
GstElement. This means less code and more uniformity with other OpenCV
elements.
The opencv element includes were full of duplicates and uneeded headers.
For example a few elements that stopped using gstcvopencvutils still
included that header file.
We know that the gchar arrays contain at least one string. Furthermore,
g_strfreev() checks if the array is NULL and simply returns if it is.
Hence, there is no need to check if the array is empty before using
g_strfreev().
CID 1327412-1327415
