Add changes file for Qt 5.12.0

Change-Id: Ic3bc5e5bc49ebc034eef8d6b6edc78a2e33c8bca
Reviewed-by: Christian Stromme <christian.stromme@qt.io>

dist/changes-5.12.0

+Qt 5.12 introduces many new features and improvements as well as bugfixes
+over the 5.11.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+https://doc.qt.io/qt-5/index.html
+
+The Qt version 5.12 series is binary compatible with the 5.11.x series.
+Applications compiled for 5.11 will continue to run with 5.12.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+https://bugreports.qt.io/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+ - This release contains only minor code improvements.
+
+****************************************************************************
+*                            Qt 5.12.0 Changes                             *
+****************************************************************************
+
+ - [QTBUG-52114] Interpreted ARGB32 as premultiplied for GraphicsVideoItem.
+
+
+****************************************************************************
+*                      Platform Specific Changes                           *
+****************************************************************************
+
+Gstreamer
+---------
+
+ - [QTBUG-39327] Added an ability to pass custom pipeline and video sink.
+   Also registered "qtvideosink" gstreamer element to be used in pipelines.
+ - [QTBUG-69402] Emitted stateChanged(StoppedState) when recording is finished.
+
+WindowsAudio
+------------
+
+ - [QTBUG-63492] Fixed crackling at the end of playback.
+ - [QTBUG-45174] Introduced QT_WAVE_BUFFERS env var to define numbers of buffers.
+
+DirectShow
+----------
+
+ - [QTBUG-32743] Implemented image capture settings.
+ - [QTBUG-53019] Added option to allow building without evr support.
+   The environment variable QT_DIRECTHSOW_NO_EVR can now be used to disable EVR at runtime.
+
+Android
+-------
+
+ - [QTBUG-61115] Implemented QMediaPlayer::setPlaybackRate().
+
+IMX
+---
+
+ - [QTBUG-50927] Copied data and uploaded back if mapping has not succeeded.