Add changes file for Qt 5.12.9

+ 9cabb018 Bump version
+ aced4ce5

 Assistant: keep text at the top visible when resizing

Change-Id: I73b5f131e4bbb3f9a62db6707fc0a8b9e241baca
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

dist/changes-5.12.9

+Qt 5.12.9 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.12.0 through 5.12.8.
+
+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.