Add changelog for Qt 5.4.0 release

Change-Id: I4828f4d3d66dd4f6739388d10e91395ed951f3b5
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Merge remote-tracking branch 'origin/5.4.0' into 5.4

Change-Id: Ia189ec48c4cbbdec65f87d9de1bfe6aac88ef56e