[Plot] Icons for plot controls

Add icons for plot pan/zoom controls, to step
back through or undo pan/zoom states. WTD-533.

platform/features/plot/res/templates/plot.html

@@ -73,7 +73,7 @@
                ng-click="plot.stepBackPanZoom()"
                ng-show="plot.isZoomed()"
                title="Restore previous pan/zoom">
-                <span class="ui-symbol icon">B</span>
+                <span class="ui-symbol icon">&lt;</span>
             </a>
 
             <a href=""
@@ -81,7 +81,7 @@
                ng-click="plot.unzoom()"
                ng-show="plot.isZoomed()"
                title="Reset pan/zoom">
-                <span class="ui-symbol icon">R</span>
+                <span class="ui-symbol icon">I</span>
             </a>
 
         </div>