Update to 2.2.18

Changelog:
Version 2.2.18 (9 June 2019)
--------------

	When compiling for GTK-4, require version >= 3.96.0 and make
	consequential changes including to documentation (acinclude.m4,
	README; doxygen.main.in, application.h, extension.h,
	file_print_manager.h, file_print_manager.cpp, gobj_handle.h,
	notifier.h, prog_present.h, reassembler.h, shared_handle.h,
	text_print_manager.h, text_print_manager.cpp, thread.h, widget.h,
	window.h and window.cpp; tests/test_application.cpp;
	tests/extra/test_file_print_manager.cpp,
	tests/extra/test_text_print_manager.cpp).

	Update other documentation (doxygen.main.in; async_queue.h,
	gerror_handle.h, shared_handle.h).

Version 2.2.17 (24 October 2018)
--------------

	Set FD_CLOEXEC on pipe file descriptors for Notifier objects
	(notifier.cpp).

	Add support for guile-3.0 (acinclude.m4; extension.h).

Version 2.2.16.1 (27 June 2018)
----------------

	Provide a configuration file suitable for doxygen-1.8.14; include
	generated .js files in the distribution tarball; rebuild the
	documentation (doxygen.main.in; docs/Doxyfile.in,
	docs/Makefile.am).

Version 2.2.16 (27 June 2018)
--------------

	When compiling for GTK+-4, require version >= 3.94.0
	(acinclude.m4, README; doxygen.main.in).

	Fix changes in GTK+-3.94 (file_print_manager.h,
	file_print_manager.cpp, window.h, window.cpp).

	Fix the change from GdkPixbuf to GdkTexture in GTK+-3.93
	(file_print_manager.h, file_print_manager.cpp, window.h,
	window.cpp).

	Let the CGU_USE_GTK manifest constant in cgu_config.h indicate the
	version of GTK+ that the library is configured for, and set
	GDK_VERSION_MIN_REQUIRED for GTK+-4 (acinclude.m4;
	tests/test_application.cpp,
	tests/extra/test_file_print_manager.cpp,
	tests/extra/test_text_print_manager.cpp; cgu_config.h,
	text_print_manager.cpp).

	Correct documentation (application.h, extension.h, window.h).


(ryoon)