feature request: improve Save/Export

[guiv42]

TuxGuitar 1.6.3 (behavior identical to 1.5.6)

Saving file under foreign format can lead to silent data loss

Open a file with a foreign format (i.e. not a TuxGuitar native file). Edit file, then Save: TuxGuitar keeps by default the original file format. This can lead to silent data loss, if the foreign format does not handle correctly the modifications. Problem already seen several times by users: lyrics lost when using gp3, #447, and there are other similar old tickets in SF. Many applications issue a warning when user tries to save a file under a foreign format

Export under foreign format

Imagine song "A.tg" is open. File/Export under another format, let's say "B.xx"

The usual behavior of an Export function is:

• save file "B.xx" to disk
• keep active file "A.tg"

But TuxGuitar does:

• save file "B.xx" to disk
• set "B.xx" as the active file

I find this very confusing: link with original file "A.tg" is lost. Following call to "Save" function opens "Save As". To restore the link with the original "A.tg" song it is required to overwrite it.

I'll try to understand how this works, while preparing future file format evolution

[guiv42]

todo:

• [x] save under foreign format, warn user (edit: "Save" and "Save As" only save .tg files, use "Export" for other formats, see #470, #481)
• [x] export, keep current file active (done, #467)