FileInfoPropertyProvider and extmetadata improvements

[CXuesong]

c.f. mw:API:Imageinfo

Here are some suggestion on what we can do more based on #74

• Some of the ExtMetadata are actually well-known so we may consider exposing them in a early-binding fashion
  https://github.com/wikimedia/mediawiki/blob/a638c0dce0b5a71c3c42ddf7e38e11e7bcd61f7a/includes/media/FormatMetadata.php#L1794-L1817
  so consumer can access them with something like this

WikiPage wp;
// ...
wp.LastFileRevision.ExtMetadata.DateTime.Value    // wp.LastFileRevision.ExtMetadata["DateTime"].Value
wp.LastFileRevision.ExtMetadata.ObjectName.Value

• Add support for iiextmetadatalanguage, iiextmetadatamultilang, and iiextmetadatafilter in FileInfoPropertyProvider.