Export tool doesn't question me about replacement of existing sql-file

- Select file you want export to (for example file doesn't exist)
- make export
- close Table tools dialog
- open it again (field with file name already filled with existing file name, that was previously created)
- press EXPORT button - it will replace file without question

I think that must be flag, that must be set when user selects file name from standard `Open File` dialog and reset when dialog is closed or after successful export. And when user presses EXPORT button `heidisql` will check this flag and question user if it's not set.

Though current behavior is comfort too, but it's not safe.