Fix partial misquoting in croak usage

Thus fixing parsing with perl_checker.

Maybe it would be better to use qq() here ...
Do not use 'defined' on an array

Spotted by Thierry Vignaud.
Add overrides for Gtk3::Image

Based on a patch by Dave M.
Add two more tests for Gtk3::TextBuffer

Namely for:
* textiter->forward_find_char
* textiter->backward_find_char
These both involve callbacks with GI_TYPE_TAG_UNICHAR, support for which was
added to Glib::Object::Introspection recently.