Added movie volume animation control to <image> and <stereo_image> tags to be used in the form:

    <slide>
        <properties>
            <property name="volume" type="float">0.20</property>
        </properties>

        <property_animation>
            <key_frame time="0.0">
                <property name="volume" type="float">0.0</property>
            </key_frame>
            <key_frame time="2.0">
                <property name="volume" type="float">1.0</property>
            </key_frame>
            <key_frame time="10.0">
                <property name="volume" type="float">1.0</property>
            </key_frame>
            <key_frame time="12.0">
                <property name="volume" type="float">0.0</property>
            </key_frame>
        </property_animation>

        <layer>
              <image volume="$volume" looping="ON">big_buck_bunny_1080p_stereo.ogg</image>
       </layer>
    </slide>