Config switch: Use only one MT resource for InstantTranslate file translations

For a file translation in InstantTranslate it makes usually no sense, to query more than one MT resource.

Therefore a new InstantTranslate-specific config switch should be introduced: "Query only one MT-resource for InstantTranslate file translation"

By default this switch should be "true" (backwards-incompatible change, that must be part of the important release notes).

If the switch is true, the MT match resource with the highest configured default match rate is used.

If multiple have the same default match rate, than the first one should be used.