Add GPL exception to license.

These changes adapt the sources to the GPL exception text that allows non-GPL applets, workflows, plugins.

I also added a menu item that displays the new license. If ilastik is running from a binary, the detailed license used in this dialog is taken from the COPYING file in the binary.

I still need to provide a patch to buildem (both linux/osx  and windows) to put an appropriate COPYING file in the binary.