The virtual keyboard in the Greeter allows to type only a limited set of keys

Originally created by @intrigeri on #12385 (Redmine)

Thanks to #5563 (closed) we now have a screen keyboard in the Greeter. That’s a pretty good start!

Now, it supports only a few keyboard layouts, and even those are partial: e.g. when one selected French language and keyboard, I could not find any way to type “é”. So users who need a screen keyboard won’t be able to type the passphrase to unlock their persistent volume if it includes “special” characters.

Related issues

• Related to #8281 (closed)
• Related to #5563 (closed)
• Related to #8444 (closed)