Fix fullscreen support with GStreamer

Fullscreen support has not been working with GStreamer since Qt 4.8,
the issue was the native fullscreen window was not fully ported, and
was affected by some bitrot.

This patch fixes the native fullscreen window for GStreamer, ensures the
fullscreen options are available on WebKit1, and adds a context menu and
web action to toggle fullscreen mode.

Change-Id: I9c6c5a1e36fbbc8e1d1fae371bdf9abc1aee7775
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>