Include option for search Original or Translations only

[Zodiac1978]

If you want to find a bad translation and correct it, it would be very helpful to filter just the translations or Original strings.

For example "email" is translated to "E-Mail" in German, but many people use words like "email" or "Email" or "e-Mail". If I want to correct this, the "Any" filter is no help, because it is searching in the original string too, which gives me all strings which contain "mail", but I just want to find the bad spelling.

First reported here: https://glotpress.trac.wordpress.org/ticket/152