Keyboard shortcuts now work when toolbar is set to None in matplotlibrc

Following from issue #1829, I've implemented a fix for keyboard shortcuts when the toolbar is not displayed.

In the existing code, state/logic of pan/zoom tools are coupled to the existence of a toolbar, so I went for the easiest fix possible: keeping the toolbar but setting visibility to false when None is chosen in matplotlibrc.

So far it's only implemented in the Qt and GTK backends: I don't have the others so no way to test.