Use dedicated window to access container shell

[MarvinWeitz]

Until now the container shell is accessed by executing %shell in a cell. After this, all commands will be executed as a bash command in the container until %exit is executed. This comes with the drawback of not having a "shell feeling" and the commands polluting the notebook's cell.

[MarvinWeitz]

This commit implements a frontend button that opens a shell-like instance of the Notebook based on this tutorial and this example extension. This is still not the desired solution. In another attempt, I will use the interactive flag of a docker run command.

[MarvinWeitz]

This comit implements a better version of the shell. It uses signals to consistently enter the initial code to access the docker container's shell.