libgimpwidgets: don't leak the empty_iter in GimpIntStore
libgimpmodule: disconnect from the modules' "modified" signal

when GimpModuleDB is finalized.
libgimpconfig: don't leak "expanded" in gimp_config_path_expand_to_files()
app: don't leak the gimpdir GFile in app_run()
app: don't leak all results from separate_uline_pattern() in GimpSpinScale
app: don't leak "buffer" in gimp_interpreter_db_add_binfmt_misc()
app: free the default brush, pattern etc in gimp_finalize()

and print a warning if there are any contexts left in gimp->context_list.