player: write title to watch_later file

write_title() writes the media title to the media's watch_later file
as a comment if 'write-filename-in-watch-later-config' is set. Prior to
this change, setting the option would only write a commented filename at
the top of the file. Now the media title is commented below the
filename, with the prefix "title: "

Before:
 # https://example.com/watch?v=foobar
 start=35
 gamma=1

After:
 # https://example.com/watch?v=foobar
 # title: Foo Bar (HD Video)
 start=35
 gamma=1

Author: yatinlala

player/configfiles.c

@@ -265,6 +265,19 @@ static void write_filename(struct MPContext *mpctx, FILE *file, char *filename)
     }
 }
 
+static void write_title(struct MPContext *mpctx, FILE *file)
+{
+    if (mpctx->opts->write_filename_in_watch_later_config) {
+        const char *title = mp_find_non_filename_media_title(mpctx);
+        if (title) {
+            char write_title[1024] = {0};
+            for (int n = 0; title[n] && n < sizeof(write_title) - 1; n++)
+                write_title[n] = (unsigned char)title[n] < 32 ? '_' : title[n];
+            fprintf(file, "# title: %s\n", write_title);
+        }
+    }
+}
+
 static void write_redirect(struct MPContext *mpctx, char *path)
 {
     char *conffile = mp_get_playback_resume_config_filename(mpctx, path);
@@ -337,6 +350,8 @@ void mp_write_watch_later_conf(struct MPContext *mpctx)
 
     write_filename(mpctx, file, path);
 
+    write_title(mpctx, file);
+
     bool write_start = true;
     double pos = get_playback_time(mpctx);