Require Gtk >= 3.24, add TableCell section in interface view
requested to merge michaelweghorn/accerciser:michaelweghorn/add_tablecell_in_interface_view into master
This adds a section for the TableCell AT-SPI interface to the interface viewer.
It also bumps the Gtk requirement to 3.24, mainly to address warnings I saw when editing the existing .ui file with glade, s. commit message for details. (There's no direct dependency between requiring a newer Gtk version and being able to make use of the TableCell interface.) If requiring that Gtk min version (released about 5 years ago) is undesirable, that requirement can be lowered (or split into a separate change).