UI: Don't load global plugins in portable mode

Portable mode is meant to be separated from the rest of the system, and
as such it doesn't make sense to load globally installed plugins there.

On Windows, there currently are only two major plugins that install
themselves there. Plugins installing themselves there is good, but it
currently also means that even portable instances load them which can
make testing and debugging in a clean environment annoying.

UI/window-basic-main.cpp

@@ -178,6 +178,9 @@ static void AddExtraModulePaths()
 				    data_path_with_module_suffix.c_str());
 	}
 
+	if (portable_mode)
+		return;
+
 	char base_module_dir[512];
 #if defined(_WIN32)
 	int ret = GetProgramDataPath(base_module_dir, sizeof(base_module_dir),