Exclude markdown mapped documentation files from Files list

The files with an obvious documentation extension are excluded from the "Files list" (e.g in top blue bar).
Files that are mapped through `EXTENSION_MAPPING` to markdown files are also documentation files and should also be excluded.

src/filedef.cpp

@@ -1918,7 +1918,8 @@ bool FileDefImpl::isDocumentationFile() const
          name().right(4)==".txt" ||
          name().right(4)==".dox" ||
          name().right(3)==".md"  ||
-         name().right(9)==".markdown";
+         name().right(9)==".markdown" ||
+         getLanguageFromFileName(getFileNameExtension(name())) == SrcLangExt_Markdown;
 }
 
 void FileDefImpl::acquireFileVersion()