WIP: Allow to unfold keyboard to allow for non-numeric passwords

Since this comes up ever so often this MR does:

• add a keyboard button to the lock screen dial pad that unfolds the keyboard on press
• allow arbitrary text input
• hide keyboard when we time out and switch back to the lock screen

Design issues

• does this fly at all?
• should we hide the keypad with a revealer when the keyboard is folded out?
• should the label change from passcode to passord ?

TODOs (mostly mechanical):

• replace keypad-button by regular buttons
• adjust d/copyright
• add to docs
• add keypad tests from libhandy too

/cc @tobias.bernard

Closes: #97