ui/widget: Make template ver# warning more useful

Composite widget templates are only supported by GTK+ 3.10 and up, but if a user targeted a lower version, we only told them that said version did not support templates, but provided no help on how to resolve that.

A user in #gtk+ mistook that vague warning as meaning widget templates had been deprecated in their (then) target version, GTK+ 3.0. Whoops!

It makes more sense to tell users what version they need, which is new info, instead of what they are currently targeting, which they know.