On Mint+Cinnamon, openpgp-applet's output dialogs don't appear in window list or alt-tab and thus can be mistakenly left open

See https://gitlab.tails.boum.org/tails/tails/-/issues/17780

The issue seemed to be that MessageDialogs without a visible parent window don't behave correctly. I changed it to a regular GTK window and now they appear in alt-tab on my machine (Mint 19.3) and appear in the window list. It's very easy to leave one open without this patch potentially exposing sensitive data.

Patch attached. Been using for several days, no issues.fix_output_dialog.patch