Fix that D source files should not open in C++ editor

The freedesktop.org specification for some reason makes it a subclass of
C sources, so override the MIME type as a workaround.

Task-number: QTCREATORBUG-20539
Change-Id: Ie822fd796c8b3b570ecb840412d74a12b3c1d5a7
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/CppTools.json.in

@@ -113,6 +113,13 @@
         \"        </magic>\",
         \"    </mime-type>\",
 
+        \"    <mime-type type=\'text/x-dsrc\'>\",
+        \"        <comment>D source code</comment>\",
+        \"        <sub-class-of type=\'text/plain\'/>\",
+        \"        <glob pattern=\'*.d\'/>\",
+        \"        <glob pattern=\'*.di\'/>\",
+        \"    </mime-type>\",
+
         \"</mime-info>\"
     ]
 }