Add ToolbarView entry

[AkshayWarrier]

https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/class.ToolbarView.html

[sonnyp]

We might want to consider droping the actionbar demo in favor of this one.

Let's have 2 dropdowns in the demo where the user can choose what type of bar to use for top/down.

see the list of supported "bars" in libadwaita documentation

Please don't put it in a dedicated window, it should render directly in Workbench preview