glossary: prompt source / target term when adding manually

Currently, one way to enter new terms to the glossary file is manually adding a blank entry through the glossary tab, and then editing it. This means that the glossary file saves these blank entries (autosave).

This change fixes the way in which new entries are added manually, prompting the user for an English and Target terms. A new row is added and saved if either the English or Target term is entered, as it means that the user is actively trying to add a non-blank entry.