FEAT: on-drop-files event handler on Windows

[Oldes]

With this modification it is possible on Windows drag files from OS and drop them into Red opened windows. Result is an event with block of file names.

See tests\view-drop-files.red file for basic usage example.

So far it is only for Windows... other systems will just ignore this event so nothing will be fired there.

[greggirwin]

This would be a really nice feature on Windows. PR needs an update, because of other new events, but the main question is whether @dockimbel wants to include it, as it's Windows-only. I think it's valuable enough to include.

[endo64]

It would also be nice to have on-drop-file event for specific widgets instead of whole window.

[greggirwin]

In the grand scheme, we need to think about the DnD system, where drag-start can set info for the drag object, which can automatically be checked in the drag event, and easily map to face cursor changes, to indicate droppability. This then leads to generalized eventing/reactive handling, for which a number of us have experiments I believe.

[greggirwin]

If we can find someone to champion this on MacOS, we can eliminate the platform specific issue.

[greggirwin]

@qtxie @dockimbel, this has conflicts now, but I'd like to see it resolved one way or another.

[greggirwin]

If we don't want to do it this way, but full I/O brings us something like R2's system port, people can add the functionality that way.

[greggirwin]

Since we don't run on MacOS right now, let's get this in place. :^) @qtxie @dockimbel ?