Enhancement: save the original subtitle between line changes

[zedqwee]

When translating text, it's really annoying when "Show Original" is checked, and when switching between lines, that original changes to the line it currently is. Because sometimes you reread your translation and want to correct something, and the original "untranslated" phrase is no longer there. As a solution I suggest to either save the Original at the first opening and not to change it, or to make a manually load, so users can select a separate file for the original