Add capability for KeyboardEvent.code + capture

[vidartf]

This:

• Adds the capability for KeycodeLayout to also use the code field from KeyboardEvents. It still allows for adding keyCode values to the map, both for backwards compatibility, but also since there are some keys that will not have a code value (it will be either an empty string or "Unidentified").
• Adds a map for code fields for EN_US.
• Adds a package with a widget for capturing keyboard layouts.
• Adds an example app for capturing keyboard layouts.
• Adds a fallback for mapping the key field from KeyboardEvents onto the list of "Special Keys" from the spec. This is only considered if there are no match for the event via the code/keyCode fields.

Resolves #74.

TODOs:

• Add tests for capture widget / example.
• Find a way to explicitly prompt for a key value for "Dead" keys. This might need some extra thought to catch corner cases. We should not try to capture Dead keys, as they are unsuitable for use in shortcuts.
• Seek out corner cases for layouts where isComposition is relevant. Capture any behavior in tests.
• Figure out a pattern for adding other layouts. Should they be added to keyboard package, or to one or more separate language packs?

[fcollonval]

Here is what I got using a fr-fr keyboard:

fr-fr layout

codes: {
  "AltLeft": "Alt",
  "AltRight": "AltGraph",
  "ArrowDown": "ArrowDown",
  "ArrowLeft": "ArrowLeft",
  "ArrowRight": "ArrowRight",
  "ArrowUp": "ArrowUp",
  "Backquote": "²",
  "Backslash": "*",
  "Backspace": "Backspace",
  "BracketRight": "$",
  "Comma": ";",
  "ControlLeft": "Control",
  "ControlRight": "Control",
  "Delete": "Delete",
  "Digit0": "À",
  "Digit1": "&",
  "Digit2": "É",
  "Digit3": """,
  "Digit4": "'",
  "Digit5": "(",
  "Digit6": "-",
  "Digit7": "È",
  "Digit8": "_",
  "Digit9": "Ç",
  "End": "End",
  "Enter": "Enter",
  "Equal": "=",
  "Escape": "Escape",
  "F1": "F1",
  "F10": "F10",
  "F11": "F11",
  "F12": "F12",
  "F2": "F2",
  "F3": "F3",
  "F4": "F4",
  "F5": "F5",
  "F6": "F6",
  "F7": "F7",
  "F8": "F8",
  "F9": "F9",
  "Home": "Home",
  "Insert": "Insert",
  "IntlBackslash": "<",
  "KeyA": "Q",
  "KeyB": "B",
  "KeyC": "C",
  "KeyD": "D",
  "KeyE": "E",
  "KeyF": "F",
  "KeyG": "G",
  "KeyH": "H",
  "KeyI": "I",
  "KeyJ": "J",
  "KeyK": "K",
  "KeyL": "L",
  "KeyM": ",",
  "KeyN": "N",
  "KeyO": "O",
  "KeyP": "P",
  "KeyQ": "A",
  "KeyR": "R",
  "KeyS": "S",
  "KeyT": "T",
  "KeyU": "U",
  "KeyV": "V",
  "KeyW": "Z",
  "KeyX": "X",
  "KeyY": "Y",
  "KeyZ": "W",
  "Minus": ")",
  "Numpad0": "0",
  "Numpad1": "1",
  "Numpad2": "2",
  "Numpad3": "3",
  "Numpad4": "4",
  "Numpad5": "5",
  "Numpad6": "6",
  "Numpad7": "7",
  "Numpad8": "8",
  "Numpad9": "9",
  "NumpadAdd": "+",
  "NumpadDecimal": ".",
  "NumpadDivide": "/",
  "NumpadEnter": "Enter",
  "NumpadMultiply": "*",
  "NumpadSubtract": "-",
  "PageDown": "PageDown",
  "PageUp": "PageUp",
  "Period": ":",
  "Quote": "Ù",
  "ScrollLock": "ScrollLock",
  "Semicolon": "M",
  "ShiftLeft": "Shift",
  "ShiftRight": "Shift",
  "Slash": "!",
  "Tab": "Tab"
}
modifiers: ["Alt", "AltGraph", "CapsLock", "Control", "Shift"]