kcms/baloo: Fix currently indexed file name label component

1. file name argument was missing from i18n call;
2. Baloo.Monitor does not have an upper-cased `FilePath` property;
3. Extra check for filePath is not required, as by default it's just an
   empty string.
4. The whole item was outside of FormLayout, but still has
   Kirigami.FormData.label property set.