Add changelog for Qt 5.4.0 release

Change-Id: I6aa15e1423fa802fffac852bd81860f569bcf9fd
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@theqtcompany.com>
Merge remote-tracking branch 'origin/5.4.0' into 5.4

Change-Id: I06ee8fd7549b6028e2fd2e81ffcbcb90b76683a1