Build cleanup: bump glib and gtk requirement
Let's eliminate all our glib and gtk version ifdefs.
The new requirements are glib 2.44 (March 2015) and gtk 3.20 (March 2016).
We require newer meson (although for compiling only) and fribidi (although optional), anyway.
gnome-terminal is about to bump these two (to even newer versions I think) in gnome-terminal#104 (closed).
Indended for master only, no 0-58 backport.