nmbug.js: Add a 'frame' argument to nmbug.show

nmbug.js: Add a 'frame' argument to nmbug.show

It defaults to the root window, but handlers can override it if it
makes sense to launch the dialog in a particular frame.  JavaScript
objects are also scoped within a given window, so we need to attach
the dialog polyfill scripts to the window that will be getting the
dialog, and use frame.dialogPolyfill to register it.