Qubes: Stable integration

[apyrgio]

For stabilizing Qubes OS integration, Dangerzone on Qubes should be on feature parity with the rest of the platforms. This means also implementing support for:

• [ ] https://github.com/freedomofpress/dangerzone/issues/433
• [ ] https://github.com/freedomofpress/dangerzone/issues/444
• [ ] https://github.com/freedomofpress/dangerzone/issues/447
• [x] https://github.com/freedomofpress/dangerzone/issues/436
• [ ] #529
• [x] #559
• [ ] https://github.com/freedomofpress/dangerzone/issues/530
• [ ] #567
• [x] #574
• [ ] #577

Also, in order to mark the Qubes integration as stable, we need to run QA tests for this platform successfully.