GTK sources say:
  Deprecated: 3.12: Style contexts are invalidated automatically.

https://www.eclipse.org/swt/faq.php#gtkstartup says:
  Eclipse/SWT 4.16.x: GTK+ 3.20.0 and its dependencies
  Eclipse/SWT 4.15.x: GTK+ 3.14.0 and its dependencies

This means that it's been at least 3 years since SWT requires GTK 3.20.
Sounds safe to stop using the deprecated function now.

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>