Iframe refactor

[Raruto]

Follows: #528
Precedes: #523

Motivation

Streamlines iframe development and testing by:

• avoiding setting up an external LAMP server for testing (at least for basic stuff)
• move all the related code into src/services/iframe.js

New map control: "Open in Iframe" (dev mode)

How to test

Add the following within your local config.js:

const G3W_IFRAME = {                  // testing MESSAGE sent to "Open in iframe" map control
  id: null,
  action: 'app:getcenter',            // or 'app:getextent'
  data: { epsg: 4326 }	
};

...

devConfig() {
  g3wsdk.gui.GUI.once('iframe:message', (w, e) => { w.postMessage(G3W_IFRAME, '*') });
}