Prefix chapter title with chapter index

I also tweecked a 'quick & dirty' numbering mod.

```python
def create_chapter(self, chapter, i) -> bool:
start = chapter["start_time"]
end = chapter["end_time"]
diff = end - start
args = [
"ffmpeg",
"-i", self.file.media_filepath,
"-ss", str(start),
"-t", str(diff),
"-acodec", "copy",
"-vcodec", "copy",
self.file.output_chapter_file(str(i) + "-" + chapter["title"])
]
ret = subprocess.run(args)
return ret.returncode == 0

```

Called as follows. A bit ugly.

```python
def split_into_chapters(self):
if not self.file.chapters:
self.log("No chapters available.")
return False

if not self.file.create_output_folder():
self.log("Failed to create output folder.")
return False

for i, chapter in enumerate(self.file.chapters):
if not self.create_chapter(chapter, i):
self.log("Failed to create chapter " + i + " file: " + chapter["title"])
return True

```

Locate create_chapter and split_into_chapters in the Python project, then read how chapter metadata and output paths are handled. Run the existing chapter-splitting flow and verify that generated chapter filenames begin with their chapter index while splitting still succeeds.

Written by the indexing model from the issue text.