Overwrite `Panel._addToPanelBox` of Gnome Shell `js/ui/panel.js` with a
custom method, which handles top bar item additions to make sure that
they are added in the correct position.
Move the valid box order creation into an own method.
This makes the `_orderTopBarItems` method cleaner and allows other
methods to easily create valid box orders as well in the future.
Introduce box orders, which allow this extension to save the order top
bar items should be in.
On extension enable, save new top bar items to those box orders.
I'm using version 1 for the first development phase (like what major
version zero (0.y.z) is in Semantic Versioning 2.0.0), so that the first
production version then gets version 2.
