Instantiating a template when the runtime GTK version is too old

[martinling]

Discovered while testing greatscottgadgets/packetry#239.

The UI file specifies:

<requires lib="gtk" version="4.12"/>

When run on a Windows test system that still had 4.10.5 DLLs in the PATH, the program panics with:

Failed to retrieve template child. Please check that all fields of type `GtkButton` have been bound and have a #[template_child] attribute.

In this case the message is misleading, as the problem is not with a field mismatch, but with a version mismatch.

This is related to #989, but the issue here is with runtime version checking, rather than compile-time checking of the template.