Rename the 'Cancel' button to 'Close', which is what it is named in the other KIPI plugins.

Rationale:
- When user tries to export some picture(s), he will be presented with this dialog.
It's OK for the button to be 'Cancel' here, and 'Close' would be equally fine.

- After the user has exported picture(s), he will be back to this dialog. The user
does not want to 'Cancel' anything, he has already done what he wanted. Now he
wants to get back to the application, so the button should be 'Close'