tails-greeter: support RTL

tails-greeter should switch to/from right-to-left when selecting a RTL / LTR language.

Currently, when one selects a RTL langage, the persistence passphrase window moves to RTL, but if one selects a LTR language back, it stays right-aligned.

Parent Task: #8230 (closed)

Subtasks

#11646 (closed)
#11647 (closed)
#12056 (closed)

Original created by @tails on 5318 (Redmine)

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