Add an application tab for "shell" commands

[vkottler]

Since we're thinking seriously about adding a new kind of application tab / UI widgeting system on #252, at this point we may as well integrate certain "shell" style interfaces like an ipython console, or maybe just a regular console to run shell commands (probably not worth trying to emulate a real terminal.. or is it?)

[vkottler]

Something like this would need a UI mock-up. stdin + stdout + stderr, buttons to send signals etc.

[vkottler]

if ssh + tmux can solve it this is not really necessary