Discussion PR - AppMenu + python3

[koreno]

Hi! I've added an 'App' layer on top of termenu that allows it to act as a pretty dynamic and rich terminal-based menu application. You can go in and out of sub-menus, preserving selections at each level, invoking actions on selected items, all in OOP/functional style. Admittedly, it is not as straightforward as I'd like it to be (to a some extent because of the reliance on Termenu's own plugins model, and trying to keep it decoupled from the App level), but it has enabled a great UX where I've implemented it. I've provided some examples of the feature-set (examples are somewhat awkward, hopefully the idea comes across).

I'm all open to the idea of detaching it from the Termenu core and developing separately with the due credits to its inspiration.

Oh, and there's also a bunch of python2/3 compatibility fixes with this.