download saving: improve messaging/interaction

set 'config.saveByDownload' flag when using download/manual fallback for
file saving.   Added specific messages for HTML5DownloadSaveFile() and
manualSaveFile().  When config.saveByDownload is set, suppress standard
"mainSaved" message  and also bypass saving of backup, XML, and 'empty'
files (i.e., only save main file)