Merge Scripts: Warn about potential lost changes

[Myaamori]

There are at least two cases that may lead to changes being lost:

1. Unimporting lines that were changed without first exporting the changes to the source files
2. Exporting changes when the source file has been changed since the lines were originally imported
3. Any others?

2 can be solved easily enough by hashing the source file on import and export and comparing the two. Not sure about 1 (hashing the subtitles object?).