hardware acceleration configuration setting

Hardware acceleration is now correctly enabled/disabled using Ring's
config file. The setting was moved from DeviceParams/VideoSettings,
which are used for local video, to preferences under video.

The video preferences object calls serialize() on the video device
preferences, so as to not overwrite one another (they're under the
same section in the config file).

Change-Id: I8c11ea8813dee5cc514a4fa4c1bae962a14577eb
Tuleap: #1122