Prevent panic in Notebook.PrependPage.

It is valid for a notebook's page's tab label to be NULL.  This change
prevents a nil pointer dereference when passing a nil tab label to the
PrependPage method by explicitly checking for a nil interface value.
Add bindings for GtkToolbar and GtkToolItem.
Switch the Native method to return a uintptr.

This change modifies the function signature of the various Native
methods from returing a C pointer to returning a uintptr.  This change
is being made for the following reasons:

1) cgo's psuedo package qualifier is replaced with the package local
   prefix `_Ctype_'.  Because these prefixes are package local, it
   is impossible to use the returned pointer directly in data
   structures without first type switching the result with the unsafe
   package.  When the caller is not interested in using the pointer
   for other cgo calls (for example, creating a map of GObjects to
   Go structs to look up additional context from callbacks),
   returning a uintptr does not require any additional type switches,
   and more importantly does not require the use of the unsafe package.

2) When the returned pointer is meant to be used as an argument to a
   cgo function, the returned result must fisrt be type switched to a
   package-local C type using the unsafe package.  While returning a
   uintptr does not make this process any easier, it also doesn't make
   it any more inconvenient.
Add several more Container methods.
Fix a native method for the GTK 3.6 and 3.8 build.
Implement GtkEditable GInterface bindings.
Add ButtonNewFromIconName.
Implement GtkToolButton bindings.
Implement GtkSeparatorToolItem bindings.
gtk.ImageNewFromPixbuf and gdk gdk.PixbufNew binding

Fix provided by jrick: gdk.Pixbuf.GetPixels() now uses the reflect
package to return a non-copied slice of Pixbuf data

Implemented gtk.GetPixbuf binding for GtkImage objects

Added reference and runtime finalizer to Pixbuf in
gdk.Pixbuf.GetPixels() to avoid having the returned byte slice point
to freed memory.