Confirm whether to save protected documents instead of ignoring save

When infobars are present, currently Geany ignores the save command, even from the unsaved document dialog (so closing silently fails if choosing *Save*). Instead of handling just the unsaved case (as the remainder of https://github.com/geany/geany/pull/298 does), it's better to prompt the user with the infobar message and get them to confirm whether they really want to save.