Add process tree view mode

[Alexandru Macovei]

It would be useful to see which worker processes belong to which parent ones for instance.
As a related idea, maybe there could be an option for displaying the parent's PID for each process.

I propose that the tree view be a togglable alternative to the current list view, independently of filter.

That is, the user could press, for example t to switch between tree and list view, and even enable filtering while the tree view is still in effect. Similarly, it should be possible to switch between list and tree after the user is done entering the filter text.