Unify and improve batch export code

[th-we]

Code for batch export from Sibelius and MusicXML was almost identical, but slowly diverged. Now it's unified again.

Additional improvements:

• Three MusicXML extensions are exported: .xml, .musicxml and *.mxl
• Hopefully more useful error message (also for non-batch export) if file could not be written
• Final message added
• Files are processed in a predictable order (alphabetical). The order of files when looping over a folder is otherwise rather random.

The progress dialog was removed because after a split second, it is overridden by the progress dialog looping over the bars of each file anyways.

Could someone please test on a Mac before merging? Especially how the MusicXML export works.

[rettinghaus]

With the latest Sibelius version I get this:

[th-we]

Huh? That is really strange. Can you make sure that sibmei4.plg is somewhere in your Sibelius Plugins folder and that it was not accidentally unloaded?

Thanks a bunch for testing, @rettinghaus!

[th-we]

@rettinghaus If you could try again that would be superb. I pushed two more commits, which are most likely unrelated to the problem you reported, but anyways...

[rettinghaus]

So I did more tests. The first problem was indeed my fault, but now I'm getting this error

I tried different folders and also the Batch-Sibelius-Export, all with the same result.

Running Sibelius 2023.8

NB: Direct export works fine

[rettinghaus]

Sorry again, I didn't saw your latest changes. Another test round and everything seems to work, so I'll just merge. Thanks for the effort!