configure: Define the max allowed version of GTK we want to use
COPYING: Update to new FSF address and refresh contents

Refresh the GPL, LGPL and GFDL licenses to those containing
the new FSF address in the preamble.

In addition simplify all the code license comment headers,
to not include addresses or such information.

https://bugzilla.gnome.org/show_bug.cgi?id=721549