Add changes file for Qt 5.12.3

+ 0a091107

 Bump version

Change-Id: I589a834bde03c8f2980fae920f2a95f9a41baddc
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>

dist/changes-5.12.3

+Qt 5.12.3 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.12.0 through 5.12.2.
+
+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.