[GM Commands] Split GM Commands Into Separate Files (#1766)

* Split GM commands into their own files

* Code cleanup

common/file_util.cpp

@@ -64,4 +64,9 @@ void FileUtil::mkdir(const std::string& directory_name)
 	}
 	::mkdir(directory_name.c_str(), 0755);
 #endif
-}
+}
+
+bool file_exists(const std::string& name) {
+	std::ifstream f(name.c_str());
+	return f.good();
+}