The CSS editor was feeling a little sluggish, because it was
reparsing and reapplying the CSS on every keystroke. Add a small
delay, to make this feel smoother.
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
