tails-greeter: support RTL
Originally created by Tails on #5318 (Redmine)
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)