Replaced GtkRuler with GimpRuler because GtkRuler was removed from GTK 3.

* glom/utility_widgets/gimpruler/: Added several source files from
gimp, with small changes to make it build for us, to provide the GimpRuler
widget.
* COPYING: Switch to GPL version 3 (instead of 2), because the GIMP source
code uses it.
* Makefile_glom.am:
* glom/glom_developer.glade:
* glom/mode_design/print_layouts/window_print_layout_edit.[cc|cc]: Use
GimpRuler instead of GtkRuler, creating and adding them in code instead of
in the .glade file, to avoid Glade needing to know about that widget.