Support for HTML5 "multiple" flag in file inputs

These small changes enable a feather to create a file upload button that allows multiple files to be selected. This is useful for gallery-style feathers and other types of feather that want to implement bulk uploading. Feathers can enable the flag by doing `$this->setField ... "multiple" => "true"`.

Also included is a Firecrest tweak that I merged from @onmoon (becon in the Chyrp forum) that allows Firecrest's feather type labels to support multiple lines of text without messing up the shadow effect.