gnome cpp libs: all are outdated; update to latest compatible releases

[mascguy (Christopher Nielsen)]

Presently the various libs related to C++ bindings for glib2, gtk2/gtk3, etc - along with related ones like cairomm and pangomm - are all outdated. Update them to the latest possible releases, based on the current version of glib2 (presently 2.70.x).

In addition to bringing them up to parity with upstream, these updates may also fix existing issues with some ports. And CherryTree in particular may benefit, per issue:65743

Of note, since the latest releases of these have bumped the API version - from 2.4 to 2.68 for glibmm, for example - that may require creation and support of legacy releases using the previous API rev. Particularly for old or outdated ports, which simply won't work with the newest APIs.

Net-Net, this rollout will have to be carefully coordinated and choreographed. And to avoid potential widespread breakage, all dependents will need to be well-tested - and updated, if necessary - prior to rollout.