Improvements to "File>Split audio on song markers" - file location/naming and overwrite confirmation

[AlisterH]

1. It should really ask where to save the file. Even saving to the same location as the file being worked on would make more sense than the current behaviour, which saves to the current working directory (CWD).
2. It should probably name the output files based on the name of the file being worked on.
3. If the CWD is read-only then it fails, but doesn't explain why. see comment below
   EDIT:
4. It should ask whether or not to overwrite, at least if item 2 isn't fixed. Silently overwriting things is not good, especially users might not even know where it is going to save. Other things that we write without confirmation are noise.dat [EDIT: we no longer write it at all] and cdrdao.toc (using the secret functionality to append the selection to it, when pressing a) [EDIT: as the whole point of this is to append to a file, this is intended]

[AlisterH]

The secret functionality to append the selection to cdrdao.toc also operates in CWD.
Rather than "fail without explaining why" if cdrdao.toc was read-only, it segfaulted, but I have just fixed that and added an error message.

[AlisterH]

3. If the CWD is read-only then it fails, but doesn't explain why.

Similarly, cut and copy fail silently if CWD is read-only.

[AlisterH]

1. If the CWD is read-only then it fails, but doesn't explain why.

Similarly, cut and copy fail silently if CWD is read-only.

I have added error messages so it isn't silent.

[AlisterH]

Re item 3, there is a error message saying that it failed to write each file. This is probably good enough (and I suspect I implemented it after filing this ticket).