Make the platform specific scancode available

[andydotxyz]

This is a complex topic and the int will vary based on platform (macOS is strange).
Some applications need a reference to the source and we can't expose GLFW abstraction

Fixes #1523

Checklist:

• Tests included. <- we don't have any way to test glfw generated events
• Lint and formatter run with no errors.
• Tests all pass.

[MagicalTux]

We still have some issues, see https://github.com/andydotxyz/fyne/blob/fix/1523/internal/driver/glfw/window.go#L1169

• If the key name is empty (key unknown to fyne), the event won't fire. I do not think it's an issue if the event fires with keyName = "" because it just won't do anything, and it's unlikely to be any issue in terms of performance/etc. Might be good to add in the documentation a note about it.
• I was about to talk about the tab key but it seems it's solved in fyne 2.1+ with fyne.Tabbable

[andydotxyz]

I think the last update should fix your point about unrecognised events

[andydotxyz]

Review period lapsed