Begin GTK 3.10 support.

This limits symbols deprecated in 3.10 to separate files requiring the
build tag gtk_3_8 before to be included in the build.  This allows
gotk3 to build on GTK 3.10 without erroring due to deprecated symbols.

This change adds no GTK 3.10 specific wrappers.
Implement wrappers for GtkSeparatorMenuItem
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.
Add MacOS installation help
Add build tag support for GTK 3.6.
Add CHANGES.
Do not look up signal IDs by their detailed name.
Do not save detailed signal name with context.

It was never used anyways, and this prevents a leak since the name was
never being freed.
Rework how signal callbacks work.

This change modifies how (*glib.Object).Connect and IdleAdd work by
creating a GClosure.  This allows safely connecting funcs with
arguments matching those of a glib/gdk/gtk callback.

This does break the public API with previous versions.
ConnectWithData is no longer available (use Connect instead) and
callback funcs taking a CallbackContext must be changed to pass
callback parameters directly.

The examples in gtk/examples have been modified to use the new
callback behavior, and can be useful when updating applications to use
the new API.

Based of work done by dradtke on his GoTK3 fork
(https://github.com/dradtke/gotk3) after discussion on how to
integrate with Conformal's GoTK3.  Thanks!
Give pointerVal an appropiate comment.