Add changes file for Qt 5.12.0

Change-Id: Ia703f49cf0218087a7b51a63d1a86423573ed62d
Reviewed-by: Mitch Curtis <mitch.curtis@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.
+
+****************************************************************************
+*                                 Dialogs                                  *
+****************************************************************************
+
+ - [QTBUG-69095] Add a signal to the Dialog class to allow the client to
+   intercept the button presses and optionally prevent further processing
+   of the event. This allows performing some validation on the fields
+   before dismissing the dialog.