Add keyboard shortcuts to common actions in Hajk

[jacobwod]

Update

The following has been implemented so far:

• New Infoclick (MapClickViewer) Overhaul of FeatureInfo (i.e. Infoclick) #1034 implements the arrow keys shortcuts for previous/next item in list
• The new Measurer plugin Feature/1243 new measure tool #1292 implements the Esc key to finish measurment.

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Idea

The shortcuts should be global, work all the time when Hajk is in focus, except:

1. When focus is in input fields (e.g. user types search phrase)

Examples of actions that could benefit from shortcuts (in no particular order):

1. Browse results in infoclick results window. Previous ← and next → are the obvious options.
2. Pressing S anywhere should put focus in search box.
3. Perhaps shortcuts that toggle some plugins, e.g. L could toggle LayerSwitcher while P the Print plugin.
4. Esc to finish/close a measurement or draw action.
5. [feel free to add more!]

Proposed implementation

Github's HotKey, although other options are possible.

[jacobwod]

I've done some tests, one very simple in FeatureInfo.js that allows user to browse through infoclick results using keyboard arrows. It's implemented roughly like this:

componentDidMount() {
    […]

    // TODO: Optimize. De-activate when not needed.
    window.onkeyup = e => {
      var pressed = e.keyCode;
      switch (pressed) {
        case 37:
          this.changeSelectedIndex(-1);
          break;
        case 39:
          this.changeSelectedIndex(1);
          break;
        default:
          break;
      }
      console.log(pressed);
    };
  }

While this works, I think we should have a more app-wide mechanism for this.

I haven't looked further into it, but there are some libraries, such as react-hotkeys that could work.