The sidebar in the object page was using up more space than it deserves.
Replace it with a combo box in the subheader. Now that we have more space,
put the CSS node tree and the style properties in the same page again.
Add yet another tab for showing information about a widget that
does not quite fit into any of the other tabs (not a property,
not style information, etc...).
For now, we show the widget state, as well as the default and
focus widget for windows.
Add a tab that shows size groups of a widget. The properties
of the size group are available here, as well as the widgets
that are part of the size group. We highlight the widgets in
the application when their row in the inspector is hovered.
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.
Show the actions that are added to GtkApplication and
GtkApplicationWindows, as well as action groups that are
inserted elsewhere with gtk_widget_insert_action_group.
https://bugzilla.gnome.org/show_bug.cgi?id=730095
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
