Handle open-only and export-only formats better

[nielsm]

There are several subtitle file types Aegisub handles, but which it doesn't make sense to save back to, or which you wouldn't want to keep saving to.

Some examples:

• The user opens a plain text file with raw translations. The user will most likely want to time this and save it as a subtitle file. The user should be prompted for a new file name on save, instead of saving back to the text file, which will strip the timing.
• The user saves to a plain text file through Save As, this currently causes the "current file name" to change to the text file, but then the user will again keep saving to a non-subtitle format. The previously saved subtitle file name should be used for the next regular save.
• The user opens an MKS file. We can't save back to that, so the user should be asked for a new file name on first save. (Same for all other open-only formats.)

[verm]

Saving to .txt shouldn't be possible under "Save As", it should be under "Export". I've never seen an app put a file-type under save which you can only use once. That's what Export is for. The same would go for opening .txt. This way we need no special file extensions when opening a file. You just go Import -> Text, Encore, MKS. This would alwasy load into a file called Untitled unless they already have a different file started.

[nielsm]

That's a different problem, user education. Lots of people seem to think we can't output SRT because it isn't in the Save As box. The Export function seems to be invisible, or not understood.

[verm]

Actually, I didn't know we could export any formats either. I thought it would export in the format that the file was 'saved as'.

In order to see the list of supported formats you have to go past the first dialogue and even after that it's a little confusing as it just says "All supported formats ..." Then a list.

We should have a list in the first dialogue that clearly says "Choose a format" and enter a filename, the save button should bring up the dialogue to save with the filename + extension added already and the proper type selected in the drop-down.