skins2: add support for art in default skin

This patch implements art display in the default skin, in the same way as it is
implemented in qt4 interface, namely if no video is available, art is displayed in lieu of video.

share/skins2/default/theme.xml

@@ -194,8 +194,9 @@
         <Button up="sysbuttons.png.aot1.normal" down="sysbuttons.png.aot1.down" over="sysbuttons.png.aot1.hover" action="vlc.onTop()" id="main.normal.titlebar.aot1" x="26" y="3" visible="vlc.isOnTop" tooltiptext="Disable Always On Top"/>
       </Panel>
       <Panel id="main.normal.component" y="24" width="0" height="0" rightbottom="rightbottom">
-        <Image id="main.normal.component.background" image="main.png.background" resize="scale" rightbottom="rightbottom"/>
-        <Video id="main.normal.component.video" width="500" height="224" rightbottom="rightbottom"/>
+        <Image id="main.normal.component.background" image="main.png.background" resize="scale" rightbottom="rightbottom" />
+        <Image id="main.normal.component.art" image="main.png.background" resize="scale2" rightbottom="rightbottom" art="true"/>
+        <Video id="main.normal.component.video" width="500" height="224" rightbottom="rightbottom" visible="vlc.hasVout"/>
       </Panel>
       <Panel id="main.normal.controls" y="248" width="0" height="0" lefttop="leftbottom" rightbottom="rightbottom">
         <Image id="main.normal.controls.bg.left" image="main.png.controls.bg.left" action="move"/>