Lockscreen: Keyboard focus and commands, more animations

Several small improvements to design and usability of the lockscreen:

- Keyboard focus changes to switchSessionPage (typing password and issuing
  unlock requests in this view not any longer possible) and other way around
- Keyboard commands for switchSessionPage: Left/Right, Return, Enter, Escape
- Password field is cleaned on changing back to the main screen or session
- Quick fade in of input elements on invocation of lockscreen
- Animating slide out of clock and input elements when switching user, which
  is in line with the other sliding animation of the Stackview. It also helps
  to easen the transition, when the users have different themes or languages

Reviewers: davidedmundson

Tags: #plasma #vdg

Differential Revision: https://phabricator.kde.org/D3055