Implement key.name for SDL2

[dlon]

• Switch from using scan codes to (mostly) key codes.
• Implement key.name() for SDL2.
• For backward compatibility, use old strings (from SDL1) where possible. It may not cover unusual cases (some constants were removed).

Closes #533.

Questions

• Is there any reason not to use key codes instead of scan codes?
• Should the scan code constants added to .constants be kept?

[Farbfetzen]

I admit I don't fully understand the difference between keycodes and scancodes. But doesn't removing scancodes make it more annying/harder to write games for non-us keyboard layouts?

[dlon]

@BastiHz Good point.

Did pygame < 2 use scancodes? If so, I wonder why key events had a "scancode" field as well as a "key" field. If it didn't, should it not continue to use key codes and add scancodes separately?

[illume]

The SDL migration guide has a section on input:
https://wiki.libsdl.org/MigrationGuide#Input

There are these sections in the wiki.
https://wiki.libsdl.org/SDL_KeyboardEvent
https://wiki.libsdl.org/SDL_TextInputEvent
https://wiki.libsdl.org/SDL_TextEditingEvent

There is also this event: SDL_KEYMAPCHANGED which "keymap changed due to a system event such as an input language or keyboard layout change (>= SDL 2.0.4)".

What do you think about starting to add relevant documentation URLs to each of the modules or functions?

Related to this is this issue about SDL_TEXTINPUT:
#623

[illume]

Scancodes are useful when you want to use the position on the keyboard. But not what the letter is.
eg. Where Z is on a US keyboard, is where Y is on a German one.

Scancode == z, means on a German keyboard it will press where the Y key is. (theoretically, there are some weird things on some keyboards... but mostly this will be true).

To answer the question: Using key codes seems the best way to go for backwards compatibility. (there is the scancode on the event for people who want to use that) Using SDL_TEXTINPUT seems the best way going forward, to be able to handle all text input properly.

I think we should merge this?