Support key-value JSON files (Import/Export)

[fhammerschmidt]

Currently, all JSON files are structured like this:

[
  { "id": "key.name", "defaultMessage": "translation message"}
]

but sometimes the simplified format is enough:

[
  { "key.name": "translation message"}
]

E.g. the first example could be the source file and the latter could be the target one.

When importing a JSON as source (the first file that is imported), we should first check if it is a source file and if not we can assume it is the simplified format.

For the output I would like to not have another JSON-Button. Probably every column should have a dropdown component that contains a dropdown button on the right but is also just a button to export with the last used export method.