Properly handle the case where QtWidgets is not available.

It does use a config test for now, just to make it clearer.  The Mac
backend currently requires widgets, but most of the others should work
without it (just not supporting QVideoWidget/WindowControl).

The QVideoDeviceControl interface was modified to remove the QIcon
usage - it was never implemented.

Unfortunately even the QML examples need widgets for the wrapper
application, and will need to be ported to QtGui based wrapper.

Change-Id: I8a55ad5cf09ab51749510bf16f49de0bd3f0bcdb
Reviewed-by: Ling Hu <ling.hu@nokia.com>

sync.profile

@@ -36,6 +36,9 @@
 );
 # Compile tests
 %configtests = (
+    # Widgets (to make things easier)
+    "widgets" => {message => "Disabling widget related code.  To enable, make sure that QtWidgets exists."},
+
     # Windows tests
     "directshow" => {},
     "wmsdk" => {},