Extend px-map-marker-group component to support numbered marker

• Re-defined structure to support rendering numbered marker:

"marker-icon": {
  "icon-base": "custom-icon",
  "icon-type": "px-nav:unconfirmed", //also component defaults to px-nav:unconfirmed
  "icon-strokeColor": "blue",
  "icon-fillColor": "white",
  "icon-content": "1", //"icon-content": "px-nav:home" for icon marker
  "icon-options": { //currently it doesn't work, will be taken after/or part of UI-2363
    "strokeColor": "blue",
    "fillColor": "white",
  }
}

• Icon-options is only valid when an "icon-content": "px-nav:home" and not a number.
• Enhanced demo to illustrate the number markers with marker group.
• Updated release notes.
• Reference demo screenshots.