Establishing GTK3 port "rules" (and some q.s)

[mike]

We've got a GTK3 port of some of exo in SpaceFM [1] (with compatibility 
back down to GTK 2.16, and I can probably handle porting the rest 
without too much issue if that's desired. It's pretty clean with not 
too many #ifdefs and a lot of the compatibility code to handle the 
different versions is centralized here [2]. It's been working well for 
years.

[1] https://github.com/BwackNinja/spacefm/tree/master/src/exo

[2] https://github.com/BwackNinja/spacefm/blob/master/src/gtk2-compat.h