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)
 - CatalogTree (catalog and selection opened) 


I've tested all and that works as expected.

I've tested the display of homescreen popover on all devices I have on xcode (tablet,iphone) 
It was not wrong before. 
We decide to change the behavior, now timeEnabled layers always display the timestamp, before we display the timestamp only whenTimeSelector was active