add callback informing about handle creation

[tclangv]

In my use case I want to save a file which is received in byte chunks from a WebAssembly component. For this I need the information when the user has selected a destination for the file, so I can trigger the start of the transfer of the file.

Starting the transfer beforehand would be possible as well, but should the user cancel out of the save file dialog, it would result in transferred data for no purpose.

[tomayac]

@tclangv Note that I have moved this callback to the end of the arguments list, as to not break existing code.