Port Tails Installer to GTK 3

The Debian Qt team is actively working on dropping Qt 4, so I guess a package that relies on it won’t be accepted into testing/sid. Note that Qt 5 is in wheezy-backports, but pyqt5 is not.

Feature Branch: feature/8980-gtk3-installer, liveusb-creator:feature/gtk3

Parent Task: #8549 (closed)

Subtasks

#9734 (closed)

Related issues

Blocks #8557 (closed)

Original created by @intrigeri on 8980 (Redmine)

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