Update edgedetect 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.
If old opencv1-style legacy include directory is available,
this change becomes purely cosmetic (maybe will compile a bit faster).
It becomes an FTBFS fix when opencv1-style include directory is missing
(possibly because opencv package maintainer decided not to pack it).
https://bugzilla.gnome.org/show_bug.cgi?id=747705
