Implement an <Icon> element used in Overlays

[cleder]

Basic Implementation in #123 still needs tests

[cleder]

<Icon>

https://developers.google.com/kml/documentation/kmlreference#icon

Syntax

<Icon id="ID">
  <!-- specific to Icon -->
  <href>...</href>                         <!-- anyURI -->
  <gx:x>0<gx:x/>                           <!-- int -->
  <gx:y>0<gx:y/>                           <!-- int -->
  <gx:w>-1<gx:w/>                          <!-- int -->
  <gx:h>-1<gx:h/>                          <!-- int -->
  <refreshMode>onChange</refreshMode>
    <!-- kml:refreshModeEnum: onChange, onInterval, or onExpire -->
  <refreshInterval>4</refreshInterval>     <!-- float -->
  <viewRefreshMode>never</viewRefreshMode>
    <!-- kml:viewRefreshModeEnum: never, onStop, onRequest, onRegion -->
  <viewRefreshTime>4</viewRefreshTime>     <!-- float -->
  <viewBoundScale>1</viewBoundScale>       <!-- float -->
  <viewFormat>...</viewFormat>             <!-- string -->
  <httpQuery>...</httpQuery>               <!-- string -->
</Icon>

Description

Defines an image associated with an Icon style or overlay. The required child element defines the location of the image to be used as the overlay or as the icon for the placemark. This location can either be on a local file system or a remote web server. The <gx:x>, <gx:y>, <gx:w>, and <gx:h> elements are used to select one icon from an image that contains multiple icons (often referred to as an icon palette.

[cleder]

merged in #182 needs more tests