Override OnImagesChanged() to call UpdateImageListIfNecessary() even in
wxGenericTreeCtrl for now, although in the future it would really make
sense to stop using wxImageList in its implementation and just use
wxBitmapBundle directly instead.
wxMSW is the only one which really needs an image list, as it's required
by the native control.
Also update the sample, even though it doesn't look very nice because
its icons are only available in a single size, so we have to always
scale them.
ced845f006