build: Use template files for the ATK enumerations

build: Use template files for the ATK enumerations

Placing the enumeration GType inline in the Makefile makes them
unreadable, and does not allow to easily update the body of the
get_type() functions. Case in point: ATK still generates thread-unsafe
versions of the enumeration and flags GTypes.

Let's update the build to something more idiomatic, and use template
files, like every other G* library does.