Images: Replace round song placeholder images to square

vlc-ios/Images.xcassets/MediaController/song-placeholder-dark.imageset/Contents.json

@@ -2,17 +2,17 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "songdark.png",
+      "filename" : "song-placeholder-dark.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
-      "filename" : "songdark@2x.png",
+      "filename" : "song-placeholder-dark@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "songdark@3x.png",
+      "filename" : "song-placeholder-dark@3x.png",
       "scale" : "3x"
     }
   ],

vlc-ios/Images.xcassets/MediaController/song-placeholder-white.imageset/Contents.json

@@ -2,17 +2,17 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "songwhite.png",
+      "filename" : "song-placeholder-white.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
-      "filename" : "songwhite@2x.png",
+      "filename" : "song-placeholder-white@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "songwhite@3x.png",
+      "filename" : "song-placeholder-white@3x.png",
       "scale" : "3x"
     }
   ],