add a check to see if magnifying icon has already been added to icons…

[ianardavis]

… container

[ianardavis]

Experienced issue when using plugin:

using not init on load
"<script>window.CI360 = { notInitOnLoad: true }</script>" in html.

add "images360.setAttribute( 'data-amount-x', imageList.length );" in JS file

initialising and adding hotspots
window.CI360.init();
window.CI360.addHotspots( 'images360', HOTSPOTS_CONFIG );

No errors occur in window, but two magnifying icons are present. this PR adds a check in the addMagnifying function to check there is not already an icon present before adding a new one.