When going from attribute mapping to model, it makes most sense
to go directly to the data tab, and when going from an action
name to the owner, we want to show the actions tab. Make it so.
Move away from cell editing, and use a popover instead. This makes
it easier to e.g. use a color chooser - there's just not enough room
in a cell for many things.
Much of this code is adapted from tests/prop-editor.c.
The entire UI is constructed with templates, so the wrapper
constructors are never called, except for gtk_inspector_window_new,
which gets called from the GTK+ code.
Moving the inspector into libgtk lets use reuse internals without
having to add public API for everything or inventing awkward private
call conventions.
https://bugzilla.gnome.org/show_bug.cgi?id=730095
