Add support for `foreignMatchFields` parameter for FAL ( `flexform.field.inline.fal` )

This commit adds support for `foreignMatchFields` parameter.
```xml
<flux:flexform.field.inline.fal label="Image" localizationMode="keep" foreignMatchFields="{fieldname : 'myimage1'}" name="myimage" multiple="TRUE" maxItems="5"/>
<flux:flexform.field.inline.fal label="Image" localizationMode="keep" foreignMatchFields="{fieldname : 'myimage2'}" name="myimage2" multiple="TRUE" maxItems="5"/>
```

Without this parameter it is not possible to have more than one `flexform.field.inline.fal` field in one element. Without it the translation mode `localizationMode="keep"` also is buggy.

I am not sure if the `foreignMatchFields` should be required if using `flexform.field.inline.fal`. What is your opinion?
In a comment i proposed that the default `foreignMatchFields` for `flexform.field.inline.fal` should be derived from the elements name. But i am not sure how to implement that and it is not part of this commit.