Add changes file for Qt 5.15.0

+ 1b555247 Bump version
+ c172655a Fix usages of text stream operators
+ a6d3c194 qscxmlc: Avoid QHash::unite()
+ f66312ce qscxmlc: Use QHash::insert(QHash)
+ d9885834 CMake: Add Qt6 forward compatible CMake API and targets
+ 78eac0c1 Update plugins.qmltypes for 5.15
+ 61ed1a9e

 Use Qt::SplitBehavior in preference to QString::SplitBehavior

Change-Id: I96b34fbdf8077db35ed5d9e02e7c43eaa6ef7f0a
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

dist/changes-5.15.0

+Qt 5.15 introduces many new features and improvements as well as bugfixes
+over the 5.14.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 5.15 series is binary compatible with the 5.14.x series.
+Applications compiled for 5.14 will continue to run with 5.15.
+
+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.