Increase minimum required GTK versions?

[Alex]

I'd like to clean up the GTK code a bit by removing some of the obsolete
codepaths; what's the policy was on supporting obsolete toolkit
versions?

Looking at the GTK version history [1], the current minimum versions
were released 2006-07-03 and 2011-02-10. Bumping the versions to 2.14
(2008-09-04) and 3.10 (2013-09-23) would remove a good few checks. Is
this still too recent? If not, perhaps the GTK+2 requirement could be
bumped to the latest (2.24 -- 2011-01-30)?

[1] https://en.wikipedia.org/wiki/GTK#History