Manage fieldnames and content regression

[Siedlerchr]

Follow up issue to 9701e35 that reworked the "Manage field names and content" dialogue and introduced the "Automatic field editor" dialogue instead.

---

Hey, just tried this. Overall I like it! I think I already found a bug though T.T

How to reproduce:

1. Add two entries

@Article{x1,
  a = {1},
  b = {2},
}

@Article{x2,
  b = {3},
}

2. Select both entries
3. Go to two fields tab
4. From a, To b; Enable Overwrite non empty fields; Press copy value

Result:

@Article{x1,
  a = {1},
  b = {1},
}

@Article{x2,
}

Expected:

@Article{x1,
  a = {1},
  b = {1},
}

@Article{x2,
  b = {3},
}

Originally posted by @ThiloteE in #8892 (comment)

[Siedlerchr]

@HoussemNasri can you please take a look?

[HoussemNasri]

I almost forgot about it; I'll make a follow-up pr very soon.