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) 
I see what you mean, this should be done in time_compare branch.