Feature: Rectangle selection

[ezatsepin]

This PR contains implementation of the component rectangle selection that allows to select multiple components quickly with the mouse:

You can see demo of the selection logic here: https://jsfiddle.net/eugene23/z35kygc7/

Code example:

editor.on('load', function() {
  initDragSelector(editor);
});

const initDragSelector = (editor) => {
  const canvasFrame = editor.Canvas.getFrameEl();
  const target = editor.getWrapper();

  if (target && canvasFrame) {
    new editor.Utils.DragSelector(editor, target);
  }
}

[artf]

Thanks for the nice PR @ezatsepin but, at the moment, I'm not accepting anything outside of the API changes (eg. UI additions).
The rule of thumb to follow here is, that if you can make it as a plugin, it should be added as a plugin and not part of the core.

As all your code logic is contained in a simple class, you're close to releasing a nice plugin. All you need to do is to create a command for your functionality.

commands.add('area-selector', {
  run(editor) {
    // ...
    this.dragSelector =  new DragSelector(editor, editor.getWrapper());
  },
  stop(editor) {
    this.dragSelector?.destroy();
  },
});

and this is how the consumer can enable/disable it.

editor.runCommand('area-selector');
editor.stopCommand('area-selector');