[Shortcuts] Support prefix keys

[loic-sharma]

Is there an existing issue for this?

• [X] I have searched the existing issues
• [X] I have read the guide to filing a bug

Use case

Desktop apps often support keyboard shortcuts "chords" with multiple separate keypresses. Examples:

1. Visual Studio Code: Ctrl+K Ctrl+O opens a folder
2. Gmail: g i goes to the inbox

Proposal

Add a new ShortcutActivator to support this. Strawman example:

// Activated by Ctrl+K followed by Ctrl+O
const openFolderActivator = ChordActivator(
  activators: <ShortcutActivator>[
    SingleActivator(LogicalKeyboardKey.keyK, control: true),
    SingleActivator(LogicalKeyboardKey.keyO, control: true),
  ],
);

return Shortcuts(
  shortcuts: const <ShortcutActivator, Intent>{
    openFolderActivator: OpenFolderIntent(),
  },
  child: ...,
);

[flutter-triage-bot[bot]]

The triaged-desktop label is irrelevant if there is no team-desktop label or fyi-desktop label.

[justinmc]

@gspencergoog Did you have any thoughts on sequences of keys in Shortcuts when you wrote it?