videomanager: fix bad memory access

- remove videoManager global instance.
- allocate a VideoManager instance when Manager instance is created.
- add Manager::getVideoManager() method to get this instance.
- force getSignalHandlers() to be called before Manager instance creation.

All of this prevents VideoManager to access to signal handlers map,
already destroyed, causing a memory access violation.

Change-Id: I21ebff08fa8d49e797f99299de12dab023c37769
Tuleap: #413