window: make it a template class

In order to prepare ourselves for the future changes,
having the window as a template class is hugely advantageous
for we'll be able to modify the interface much more
quickly and cleanly.

This commit makes the window a template class, and only
that. No behavioral changes, nor new features were
introduced here.

https://bugzilla.gnome.org/show_bug.cgi?id=766922