Add keyboard shortcuts to the (revamped) Greeter

While I find the revamped Greeter absolutely awesome, I find it more cumbersome since keyboard navigation is more complex.

Main window:

• Alt+L: Language
• Alt+K: Keyboard layout
• Alt+F: Formats
• Alt+P: Persistence
• Alt+R: Read-only persistence
• Alt-A: Additional Settings window
• Ctrl+Enter and/or Alt+S: Start Tails

For “Additional Settings” we could have a shortcut each (which opens that option’s window):

• Ctrl+Shift+A: Administrator password
• Ctrl+Shift+M: MAC spoofing
• Ctrl+Shift+N: Network settings

Feature Branch: greeter:feature/12186-keyboard-shortcuts

Related issues

• Related to #12509 (closed)

Original created by @anonym on 12186 (Redmine)