Tails Installer should display accurate progress information instead for freezing on "Synchronizing data on disk..."

Originally created by @tchou on #10114 (Redmine)

During test sessions, Tails Installer was stucking a long moment at 95%.
I don’t kown if it’s happening often. Any feedback ?

If it’s often stucking, could we have an other way to indicate the progression ?

Feature Branch: feature/gtk3+fancy-progress-bar

Edited May 21, 2020 by tchou

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