Hide Bookmark menu for new dynamically generated playlists

The bookmark menu with the option to create a bookmark
was inadvertently displayed for new dynamic playlists,
that had no associated folder or playlist file on disk.
(e.g. after selecting some track from the database for
playback), until the playlist was modified by the user.

Change-Id: I9d6809e4d03603c651459415327f28e38162ad53

apps/playlist.c

@@ -2698,6 +2698,22 @@ bool playlist_allow_dirplay(const struct playlist_info *playlist)
     return !!(playlist->flags & PLAYLIST_FLAG_DIRPLAY);
 }
 
+/*
+ * Returns true if the current playlist is neither
+ * associated  with a folder nor with an on-disk playlist.
+ */
+bool playlist_dynamic_only(void)
+{
+    /* NOTE: New dynamic playlists currently use root dir ("/")
+     *       as their placeholder filename – this could change.
+     */
+    if (!strcmp(current_playlist.filename, "/") &&
+        !(current_playlist.flags & PLAYLIST_FLAG_DIRPLAY))
+        return true;
+
+    return false;
+}
+
 /*
  * Move track at index to new_index.  Tracks between the two are shifted
  * appropriately.  Returns 0 on success and -1 on failure.