Add Folder events

If possible, to match with the MediaGroup behavior, it would be useful to have the following events supported:

VLC_ML_EVENT_FOLDER_ADDED,
VLC_ML_EVENT_FOLDER_UPDATED,
VLC_ML_EVENT_FOLDER_DELETED,

assigned to @chouquette

changed milestone to %0.11.x

added feature label

mentioned in merge request !587 (merged)

closed with merge request !587 (merged)

closed with commit 0d40b407

mentioned in merge request vlc!1300 (merged)