overrides: support Gtk template callbacks

* Throw an error if template signal uses the 'object' attribute since we don't support GObject.signal_connect_object()
* Throw an error if template signal sets the 'swapped' attribute to 'true' since we don't support GObject.signal_connect_swapped() or @user_data
* Respect the flag for GObject.signal_connect_after()

* Throw an error if template signal uses the 'object' attribute
* Amend error messages to make it clearer they are template errors
* call set_connect_func() on the constructor object
11 jobs for gtk-template-callbacks in 33 minutes and 59 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Source Check
passed #25973
cppcheck

00:02:25

passed #25974
cpplint

00:01:00

passed #25975
eslint

00:00:34

passed #25972
fedora

00:03:11

 
  Test
passed #25976
fedora_clang

00:05:02

 
  Thorough Tests
passed #25979
coverage

00:25:05

passed #25978
installed_tests

00:06:34

manual #25982
allowed to fail manual
no_graphics
passed #25977
no_profiler

00:02:11

passed #25980
sanitizer_gcc

00:10:43

manual #25981
allowed to fail manual
valgrind