Fix memory leak in plugins window

- remove gui extension point on close, to remove the reference the plugin manager holds to the Class.

- reduce scope of var that holds reference to Gtk.Builder(). We have to be careful with
Gtk.Builder, because it can hold a reference to our Class (methods bound to signals) which creates a reference cycle
which python is not able to detect. Hence neither Gtk.Builder nor our Class are garbage collected.
Why this is not detected as a reference cylce is unclear at the moment.

There are two approaches to circumvent the problem:

1. we lose our reference to Gtk.Builder when closing the window, which lets python garbage
collect the builder, afterwards it can garbage collect our class.

2. we reduce the scope of the var that holds a reference to Gtk.Builder, so that the builder
can be garbage collected at the end of __init__.

I chose to reduce the scope because the builder is not needed class wide.