Get rid of the splash screen of Tails Installer

This is a broader objective of simplification of the workflow in Tails Installer:

• In the context of Tails, on the side “Use existing Live system ISO” it should have a button to clone exisiting Tails.
• Outside of the context of Tails it doesn’t need that cloning button.

This implies to change some parts of the code in depth: the “clone” feature is currently implemented as a command-line option, and quite a few things are done differently depending on whether it’s set or not, e.g. in the LiveUSBDialog class constructor => they will now need to be done differently dynamically, after the UI has been set up and the
target device has been chosen.

Team: u, kurono, sajolida, tchou

Blueprint: https://tails.boum.org/blueprint/bootstrapping/installer#simplify

Feature Branch: feature/8860-tails-installer-improvements installer:master,tails/stretch

Parent Task: #9005 (closed)

Related issues

• Blocks #13424 (closed)
• Blocks #14650 (closed)

Original created by @sajolida on 8859 (Redmine)