add feature extractors

[jwijffels]

https://docs.opencv.org/4.4.0/d3/df6/namespacecv_1_1xfeatures2d.html

[jwijffels]

I've added ocv_keypoints in order to eventually add SIFT descriptors where the patent was just expired but this requires xfeatures2d:

• Linking was done in Makevars.win (-lopencv_xfeatures2d) https://github.com/ropensci/opencv/blob/master/src/Makevars.win#L10
• Travis CI builds are failing because of lack of the opencv_contrib modules. Maybe @jeroen knows more how to install these on the different Travis platforms or if this is uberhaupt possible/allowed on CRAN. Otherwise, I'll have to remove this ocv_keypoints.

[jwijffels]

I've made some code conditional on the presence of xfeatures2d, working now for all builds.