Automatically Create Directory Structure on File Export

Addresses issue #281 (closed) .

Use slashes (/) in file name configuration to allow directory creation. This feature works only when saving all attachments of a data message and when creating a correspondence overview.

In other cases current behaviour has been retained (i.e. / and \ are replaced with _).

Addresses issue #293 (closed) .

When calling MainWindow::saveAttachmentToFile() the variable m_save_attach_dir was not set properly.