Separate out the initial portion of Luca's !9 (merged) Gparted gtk merge request into a separate merge request which just updates to modern Gtk2.
This will update to gtkmm 2.24. This desupports very old but still supported distributions:
Distribution EOL Gtkmm RHEL / CentOS 6 2020-Nov 2.18.2 SLES 11 2022-Mar 2.14.1
I will be added Luca's commits here as I fully review them. When complete I will then merge this request. Then I will return to the remainder of the Gtk3 port back in merge request !9 (merged).