This patch adds isScrollable() to a control, so that we can distinguish
controls that require mouse wheel (playtree, slider) from those that don't.
In the latter case, we can then forward events to vlc core.

this fixes #6457.
(cherry picked from commit f3afffbc05eea48ef29191b169859b17e3f4b994)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>