Make Tails Installer's .desktop-file translatable in Transifex

Currently one cannot translate “data/tails-installer.desktop” through Transifex.

This should be fixed by using intltool during the build process.

Only this one desktop file need to be translatable, as that is the only one we use.

Parent Task: #6470

Original created by @winterfairy on 6488 (Redmine)

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