Don't panic on valid nil argument.

Passing a nil tab label to (*gtk.Notebook).AppendPage is valid
(causing the tab to use the default label), so don't panic when a nil
interface is passed as the argument.
Add (*gtk.Window).Close wrapper.

This is the first GTK 3.10 specific function added to the gtk package.
As a build targeting GTK 3.10 is assumed, gotk3 needs to be built
using the build tag 'gtk_3_8' if using the older GTK 3.8.
Add (*gtk.Misc).{Get,Set}Alignment.
Add basic support for GtkToggleButton and GtkCheckButton.

This change adds the ToggleButton and CheckButton types, *New* funcs
for each, and the {Get,Set}Active wrapper for ToggleButton.
Add wrappers for GtkFrame and GtkShadowType.
Add GtkSeparator support.
Add GtkScrollbar and basic GtkRange support.

Fixes #16.
Add Scrollbar and Range support to cast().
Add GtkSearchEntry support.

Fixes #17.
Add Justification support to Label.
Add {Get,Set}Deletable to *Window.
temporarily allow deprecated stuff (oh and thank you gtk devs, for breaking backwards compatibility in the same major version)
merge from upstream