Add scripts to purge unused tags.
GTK3: Use accessor functions instead of direct access

GTK+ 3 removes many implementation details and struct members from its public headers.

Quote:
While it may be painful to convert, this helps us keep API and ABI compatibility when we change internal interfaces.

See https://developer.gnome.org/gtk3/3.9/gtk-migrating-2-to-3.html#id-1.6.3.3.5
FindSaxon.cmake: fix for Gentoo Linux

On Gentoo Linux, saxon.jar is installed into /usr/share/saxon-6.5/lib/, where FindSaxon.cmake isn't looking
Merge pull request #297 from darktable-org/purge-unused-tags

Add scripts to purge unused tags.
Merge pull request #286 from LebedevRI/gtk3-01-use-accessor-functions-instead-of-direct-access

GTK3: Use accessor functions instead of direct access

This seems to compile even on Debian stable, so it should be good enough for us wrt. dependencies.