inspector: Drop the button path

The button path takes up quite a bit of room, and is not _that_
useful. If this information is useful, it can find a new home
on the misc tab.
inspector: Save some more space

Instead of nesting two notebooks, use a stack switcher in
the headerbar for the outer notebook. This saves valuable
vertical space.
inspector: Fix up resource tab appearance

Now that it is not in a notebook anymore, we lost the view
class in the background. Add it back.
inspector: Add more data to resource list
inspector: Avoid a split pane for resources

There is not much room in the inspector window; squeezing a
tree and a detailed view in the same page is not great. Instead,
make the details a separate page.
inspector: Drop unused python shell code

This is getting in the way; if somebody comes by who wants
to add scripting support back, we can resurrect this.
inspector: Small reshuffling

Move the scrolled window into GtkInspectorWidgetTree.
inspector: More refactoring

Get rid of gtk_inspector_widget_tree_get_selected_object,
we can just pass the selected object along with the changed
signal.
inspector: More refactoring

Rename widget tree to object tree. It contains plenty of
non-widgets nowadays, after all.
inspector: Avoid a split pane for objects

Like for the resources page, use a separate page for details
to gain more room for both the tree and the details.
inspector: Cosmetic fixes

Expand the resource list initially.
inspector: Cosmetic fixes

Add a missing shadow in the css editor.
inspector: Cosmetic fixes

Set scrollbars and shadows consistently.
inspector: Small improvements to the resource tab

Move the close button to the header bar and center the info grid.