app-window: Turn current_item into property

We need this to be able to (un)hook on current item's state changes. Not
using automatic properties cause we need to do that from setter.

https://bugzilla.gnome.org/show_bug.cgi?id=735045
app-window: Close on machine stopping

If current machine changes state to a non-running state and window is
not main, close the window.

https://bugzilla.gnome.org/show_bug.cgi?id=735045
machine: Turn 'deleted' into a property

This is so that we can easily listen to a machine (of interest) getting
deleted from other modules.

https://bugzilla.gnome.org/show_bug.cgi?id=735045
app-window: Close on machine deletion

If current machine is deleted and window is not main, close the window.

https://bugzilla.gnome.org/show_bug.cgi?id=735045