Add Picture in Picture mode

[Resaki1]

Idea: As a user, I want to see what is going on on my board, even when scrumlr is in the background. In a PiP, I want to see the card that is currently in focus.

Example: User has scrumlr in background and is browsing github, but the card that is currently in focus is shown in a PiP

Todo:

• Check if it is even possible. Might be implemented with react-picture-in-picture, but not sure if it updates accordingly if a new card is focused
• think about what is displayed if no card is in focus