style: modified placeholder icon background colors and added folder icon

app/src/main/java/com/cappielloantonio/tempo/glide/CustomGlideRequest.java

@@ -36,6 +36,7 @@ public class CustomGlideRequest {
         Unknown,
         Album,
         Artist,
+        Folder,
         Directory,
         Playlist,
         Podcast,
@@ -60,6 +61,8 @@ public class CustomGlideRequest {
                 return AppCompatResources.getDrawable(context, R.drawable.ic_placeholder_album);
             case Artist:
                 return AppCompatResources.getDrawable(context, R.drawable.ic_placeholder_artist);
+            case Folder:
+                return AppCompatResources.getDrawable(context, R.drawable.ic_placeholder_folder);
             case Directory:
                 return AppCompatResources.getDrawable(context, R.drawable.ic_placeholder_directory);
             case Playlist:

app/src/main/java/com/cappielloantonio/tempo/ui/adapter/MusicFolderAdapter.java

@@ -42,7 +42,7 @@ public class MusicFolderAdapter extends RecyclerView.Adapter<MusicFolderAdapter.
         holder.item.musicFolderTitleTextView.setText(musicFolder.getName());
 
         CustomGlideRequest.Builder
-                .from(holder.itemView.getContext(), musicFolder.getName(), CustomGlideRequest.ResourceType.Directory)
+                .from(holder.itemView.getContext(), musicFolder.getName(), CustomGlideRequest.ResourceType.Folder)
                 .build()
                 .into(holder.item.musicFolderCoverImageView);
     }