[Extension API] Custom commands and keybindings

[huchenlei]

Extensions can now use following API to register custom keybindings:

  app.registerExtension({
    name: 'TestExtension1',
    commands: [
      {
        id: 'TestCommand',
        function: () => {
          alert('TestCommand')
        }
      }
    ],
    keybindings: [
      {
        combo: { key: 'k' },
        commandId: 'TestCommand'
      }
    ]
  })

Proper support for user keybinding customizations coming soon.