GUI v2: Welcome page

[apyrgio]

Main issue: #885

Add a welcome page, when the user starts the application for the first time. We can detect that this is the case if there are no Dangerzone settings in the user's system. The welcome page contains:

• [ ] A brief explanation of what is Dangerzone and how it works
  • The design material has a placeholder explanation of how Dangerzone wors, but we will use something more appropriate.
• [ ] A "Get started" button
• [ ] A footer with a link to a Dangerzone tutorial (see #886)

Design material

Welcome page

[harrislapiroff]

Here's a refined welcome screen concept that I think incorporates some branding while staying OS-native. (I think the welcome screen is a good place to incorporate branding!) Obviously I mocked this up with macOS in mind—we'll have to explore how realistically this aesthetic works (conceptually and technically) on other OSes:

The text content also still needs some refinement.

I additionally think the welcome screen should accept drag and drop (even if it isn't visually signaled) so the impatient can get started off the bat.