Use ng-class when possible

MInor PR, generalize the use of `ng-class` (improve code clarity)


Exhaustive list of components affected : 
 - Draw (current selected icon and current selected tool)
 - Slider (divisions)
 - Homescreen popover (when displayed)
 - Body tag
 - Import WMS (layer selected or hovered in the list)
 - LayerManager (layer has a timestamp or not)
 - Offline button  (red when ther is offline data)


I've tested all and that works as expected