app_manager: allow separated modules of Event and RyuApp

Because handler.set_ev_cls() sets observer to module of ev_cls,
Event class and RyuApp must be in same module now.

This patch let RyuApp have a list of events to be generated in app.
So, AppManager can bind sender app and receiver app via event class.

Signed-off-by: YAMADA Hideki <yamada.hideki@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>