Use "podman" when on Linux, and "docker" otherwise.
This commit also adds a text widget to the interface, showing the actual
content fo the error that happened, to help debug further if needed.
Fixes#212
To be consistent with Light Mode, the background of the
safe_extension_filename QLabel should match the adjacent QTextField,
but the text should be "grayed out"/disabled to indicate that it's not
supposed to be editable.
The user is supposed to only be able to select the safe PDF extension.
In a multi-file scenario, the extension will be the same for all files.
We follow here the design document [1]. To achieve this, we needed a
QLabel right next to a QLineEdit, to give the user the illusion that
it is the same graphical object.
[1]: https://github.com/firstlookmedia/dangerzone/files/6657536/DangerZone_NA02a.pdf
