GUI v2: Drag-and-drop interface

[apyrgio]

Main issue: #885

Update our existing drag-and-drop interface (see #409) for adding documents:

• [ ] Remove the Dangerzone logo and version
• [ ] Add "+ document" button, which should open the file browser window
• [ ] Add an explanation for what the user can do in this window
• [ ] Add the new footer (#886)

Once a document is added, the document pop-up should appear (see https://github.com/freedomofpress/dangerzone/issues/890). Once the user confirms the conversion, the windows should switch to the conversion view.

Design material

No document selected

Current way to add documents

[harrislapiroff]

Here's an idea for the drag and drop interface. If we want to maintain our branding, we can have it peeking in through the side as shown. Alternatively, we could omit that for a simple "brandless" look.

I have omitted the Dangerzone logo, but I think that's fine since it is present in the app icon.

Again, I've designed this with macOS in mind (I've had a hard time finding premade UI kits for other OSes 😢) and I imagine this won't work for all OSes. In particular the combined toolbar is a very Mac idiom and I'm not sure if Qt supports it. If not, this design could be adapted by moving the toolbar below the titlebar.