package/opencv: add qt5 support

Starting with the 2.4.6 release, OpenCV supports either Qt4 or Qt5 as GUI
toolkit, so add Qt5 in the GUI toolkit choice.

When Qt4 is enabled (and thus Qt5 is hidden and disabled), no need to
show a comment stating "Qt5 support needs Qt5", because Qt5 is not
selectable.

Conversely, when Qt5 is enabled and Qt4 is not, then no need to show a
comment stating "Qt4 support needs Qt4", because enabling Qt4 would
disable Qt5.

So, we only show the comments when neither toolkit is enabled.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: split-out the Qt5 hunk from the
 switch-selects-to-depends hunk]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>