G-API: Mat's "deownification"

* deowned Mat

* boner

* Removed canDescribe test for own::Mat

* Removed STANDALONE flag for apply() and operator()

* Removed: desc_tests for own::Mat, descr_of for own::Mat.

* Returned: tests, cv::gapi::own::descr_of; fixed alignment; Removed own::Mat's headers

* Removed unused header own/mat.hpp from gbackend.hpp