Add basic support for Video widget

Allow only for the simplest use case: to embed a video from a given
URL. The native controls are enabled by default and can be used to
start and stop playback.

Programmatic playback control and playback events are not yet fully
consistent across platforms, we'll add these incrementally.

Fix #169

Change-Id: I3429c8634964c1f4c7f1301a4c72cbbd46b3a7f1