Fix that .ui files could be opened in C++ editor

The glob "*.ui" is unfortunately not unique in the mime database, so it
falls back to using the mime magic to determine the mime type.
That process is unfortunately defined by the "standard" in a most useless
way: Even if a mime type matches both by glob and by mime magic, a mime
type that does not match by glob can be preferred, if its mime magic
matches either with a higher priority, or with the same priority but is
alphabetically "smaller" than the other mime type.

Reduce the priority of the "#include" mime magic for the text/x-c++hdr
mime type, so it is lower than the mime magic of the application/x-
designer mime type.

Fixes: QTCREATORBUG-21773
Change-Id: Idfb9ab509c13563b50381b098d643f6fa9292f27
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/CppEditor.json.in

@@ -61,7 +61,7 @@
         \"        <!-- Find include guards of header files without extension, for\",
         \"             example, STL ones like <string>. Those can have a big initial\",
         \"             comment exceeding 1000 chars, though. -->\",
-        \"        <magic priority=\'50\'>\",
+        \"        <magic priority=\'40\'>\",
         \"            <match value=\'#ifndef \' type=\'string\' offset=\'0:2000\'/>\",
         \"            <match value=\'#if \' type=\'string\' offset=\'0:2000\'/>\",
         \"            <match value=\'#include \' type=\'string\' offset=\'0:2000\'/>\",