Add changes file for 5.8.0

Change-Id: I63a99f4816101317576164d5c2335ffd9c02a86a
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>

dist/changes-5.8.0

+Qt 5.8 introduces many new features and improvements as well as bugfixes
+over the 5.7.x series. For more details, refer to the online documentation
+included in this distribution. The documentation is also available online:
+
+  http://doc.qt.io/qt-5/index.html
+
+The Qt version 5.8 series is binary compatible with the 5.7.x series.
+Applications compiled for 5.7 will continue to run with 5.8.
+
+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.
+
+****************************************************************************
+*                          Controls                                        *
+****************************************************************************
+
+ - ComboBox:
+   * [QTBUG-53921] Fixed the current text to update on model changes.
+
+ - TableView:
+   * [QTBUG-57283] Fixed a possible crash due to out of memory on ARM.
+
+****************************************************************************
+*                          Dialogs                                         *
+****************************************************************************
+
+ - FileDialog:
+   * [QTBUG-56663] Fixed selection handling when the OK button is clicked.