Remove the association between MIME types and export filters, because
LibreOffice is able to auto-detect them on its own. Instead, ask
LibreOffice to simply convert the document to a .pdf.
This association was cumbersome for yet another reason; there are MIME
types that may be associated with more than one file type. That's why
it's better to let LibreOffice decide the proper filter for the
conversion.
Our current understanding is that this change won't widen our attack
surface for the following reasons:
* The output filters for PDF documents are pretty specific, and we don't
affect the input filters somehow.
* The default behavior of LibreOffice on Alpine Linux is to disable
macros.
Closes#369
Closing windows on macOS would not actually close Dangerzone. Now that
it is a single-window program, it makes sense for it to close
immediately.
Fixes#271
Checking if files were writeable created files in the process. In the
case where someone adds a list of N files to dangerzone but exits before
converting, they would be left with N 0-byte files for the -safe
version. Now they don't.
Fixes#214
