CompilationDatabase: enable the listing of non-cpp files

Files listed in an optional "compile_database.json.files" are also
visible in the project, similar to the GenericProjectManager.

Change-Id: I1250b674298079bab90a0ffe036107a09b4ac6e5
Reviewed-by: Thomas Otto <totto@zbh.uni-hamburg.de>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/compilationdatabaseprojectmanager/compilationdatabaseconstants.h

@@ -30,6 +30,7 @@ namespace Constants {
 
 const char COMPILATIONDATABASEMIMETYPE[]  = "text/x-compilation-database-project";
 const char COMPILATIONDATABASEPROJECT_ID[] = "CompilationDatabase.CompilationDatabaseEditor";
+const char COMPILATIONDATABASEPROJECT_FILES_SUFFIX[] = ".files";
 
 } // Constants
 } // CompilationDatabaseProjectManager