Ruler is derived from deprecated GtkRuler

[Alex Valavanis]

The rulers in Inkscape are derived from the GtkRuler class, which has been removed from Gtk+ 3 and will not be replaced by an alternative.[1][2] This means that we need to either:
* Rewrite our ruler implementation to use a different parent class from another library.
* Copy the GtkRuler class into our codebase, update it to Gtk+ 3 compliance and maintain it ourselves.

[1] http://developer.gnome.org/gtk/2.24/GtkRuler.html#GtkRuler-struct
[2] https://bugzilla.gnome.org/show_bug.cgi?id=613942

[Alex Valavanis (valavanisalex)]

gimp got around this problem by adding their own drop-in replacement "gimp-ruler" class,[3] which is essentially just a fork of the original GtkRuler class. I guess making our own GTK+ 3-compliant fork is probably the way to go here, until someone hopefully creates a suitable upstream library.

[3] http://git.gnome.org/browse/gimp/commit/libgimpwidgets/gimpruler.c?id=ae67f09ddd10a2e17baf182f22ba73eded077f2a

[Alex Valavanis (valavanisalex)]

I've imported the deprecated code into lp:inkscape r11023. Please let me know if this introduced any problems.