HID: core: store the collections as a basic tree

For each collection parsed, store a pointer to the parent collection
(if any). This makes it a lot easier to look up which collection(s)
any given item is part of

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Verified-by: Harry Cutts <hcutts@chromium.org>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>