* Make modals display in source Window
this makes modals display within the window where they got opened, with the parameter `rootwindow` that, if `yes` or `true`, shows the modal always in the root TW window (`<$action-sendmessage $message="tm-modal" $param="mymodal" rootwindow="yes|true"/>`)
* pass the full event to $tw.modal.display
we need the event there to find `srcDocument` and `srcWindow`
* pass event in options object
* update modal.js to use options.event
* add docs for rootwindow tm-modal attribute
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