Split KeyPress into separate Key and Modifier types.

[gwenn]

Validate:

• Pros and cons of tuple struct versus struct ?
• Tuple struct order: (key, mods) versus (mods, key) ?
• Remove BackTab and keep only SHIFT-Tab ?
• Remove Modifiers::CTRL_SHIFT, ... ?
• Normalize:
  • Ctrl-a => Ctrl-A
  • Shift-A => A

Todo:
\E\E[ sequences on unix (we should use poll).

This PR replaces/enhances:

• Split KeyPress into separate Key and Modifier types. #333 (except \E\E[ sequences)
• Changes to allow Windows backspace keybindings #422
• Add support for using Ctrl/Alt/Meta with non-character keys #121 (should we support SUPER modifier ?)

This PR should fixes:

• Unable to do Meta-Left or Meta-Right #318
• Keybinding to ASCII characters #421

[gwenn]

@fdncred, @thomcc, @bjorn3 would you mind reviewing this PR ?
Only changes in keys.rs and lib.rs are relevant from a user perspective.
Thanks.

[fdncred]

@gwenn It looks like progress. I'm anxious to try it out. Looks like we're going to have to change some code, which is fine, assuming it works. :)