Added changes file for 5.4.0

Change-Id: Id9672b324af8aa96d79d534b616520049b6e77e3
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>

dist/changes-5.4.0

+Qt 5.4 introduces many new features and improvements as well as bugfixes
+over the 5.3.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+  http://qt-project.org/doc/qt-5.4
+
+The Qt version 5.4 series is binary compatible with the 5.3.x series.
+Applications compiled for 5.3 will continue to run with 5.4.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+  http://bugreports.qt-project.org/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+****************************************************************************
+*                           General                                        *
+****************************************************************************
+
+- Add support for reading icon variants in ICNS plugin
+- Save opaque images as opaque TIFF files
+- Add support for SubType and SupportedSubTypes options in DDS handler
+- QTBUG-42321: Fix crash in MNG reader when invoked by QIcon
+- Disable webp also for ios, as libwebp has compile issues on arm