Aduct

Element is now based on `Gtk.Grid`. Action buttons and header child is attached directly to the grid therefore.
Added functions `Aduct.Element.enable_action_button` and `Aduct.Element.disable_action_button`.

**Changes**
1. Updated docs.
2. Accessing `type` as an attribute is no longer allowed. Instead use the `get_type` method on the element or view.
3. Providers do no have `name` as attribute, it is now a GObject property and can be retrieved using `get_name` method on the provider.
4. Following (3), use GObject property to give name to providers.
5. The default name for element and views are now CSS names.