Add changes file for 4.12.4

Change-Id: I4f98bdfddfe0ba3a4d9fa9473ae91d026f453b5a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

dist/changes-4.12.4.md

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+Qt Creator 4.12.4
+=================
+
+Qt Creator version 4.12.4 contains bug fixes.
+
+The most important changes are listed in this document. For a complete
+list of changes, see the Git log for the Qt Creator sources that
+you can check out from the public Git repository. For example:
+
+    git clone git://code.qt.io/qt-creator/qt-creator.git
+    git log --cherry-pick --pretty=oneline origin/v4.12.3..v4.12.4
+
+Editing
+-------
+
+* Fixed crash when searching in binary files (QTCREATORBUG-21473, QTCREATORBUG-23978)
+
+### QML
+
+* Fixed completion of signals from singletons (QTCREATORBUG-24124)
+* Fixed import scanning after code model reset (QTCREATORBUG-24082)
+
+Projects
+--------
+
+### CMake
+
+* Fixed search for `ninja` when it is installed with the online installer (QTCREATORBUG-24082)
+
+Platforms
+---------
+
+### MCU
+
+* Adapted to changes in Qt for MCU 1.3
+
+Credits for these changes go to:
+--------------------------------
+Alessandro Portale  
+André Pönitz  
+Christian Kamm  
+Christian Stenger  
+Eike Ziller  
+Fawzi Mohamed  
+Friedemann Kleint  
+Robert Löhning  
+Venugopal Shivashankar