Express qmlvideofx effect parameters declaratively

Each effect item now exposes a ListModel property which describes the
set of user-controllable parameters.  This model is used by the
ParameterPanel component to generate a ListView whose delegate displays
the parameter name and a slider.

Change-Id: Ib26877fe434d8b746ef621e0b4adbcbe1ed9aaa8
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

examples/video/qmlvideofx/qml/qmlvideofx/EffectVignette.qml

@@ -42,11 +42,5 @@
 import QtQuick 2.0
 
 Effect {
-    // Constant properties which must be supported by every effect
-    property int numParameters: 0
-    property bool supportsDivider: true
-
-    property real dividerValue: 0.5
-
     fragmentShaderFilename: "shaders/vignette.fsh"
 }