"Open With Dangerzone" with 2 documents results in a single document getting selected in macOS

[apyrgio]

In macOS, if we select 2+ documents and attempt to open them with Dangerzone, the following will happen:

1. A single Dangerzone app will spawn.
2. A single file will be selected (the first argument, most likely)

Similar to #797, we do want to spawn a single Dangerzone instance, but that instance should convert 2+ documents.