GTK and GDK backends deprecated

The untested and broken GDK and GTK backends have been deprecated.

These backends allows figures to be rendered via the GDK api to

files and GTK2 figures. They are untested, known to be broken and

use have been discouraged for some time. The `GTKAgg` and `GTKCairo` backends

provide better and more tested ways of rendering figures to GTK2 windows.

WX backend deprecated

The untested WX backend has been deprecated.

This backend allows figures to be rendered via the WX api to

files and Wx figures. It is untested, and

use have been discouraged for some time. The `WXAgg` backend

provides a better and more tested way of rendering figures to WX windows.

CocoaAgg backend removed

The deprecated and not fully functional CocoaAgg backend has been removed