package/opencv: add a choice for selecting the gui toolkit

In OpenCV, only one GUI toolkit may be used at any one time, so group
the two existing options into a choice to make this situation explicit.

This will also be useful when we later add support for Qt5 and gtk3.

Suggested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: tweak commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>