#196 implemented setEnabled() and getEnabled()

Implemented the setter and getter for the new field enabled in
SlidableWMSServiceLayerGroup. This also enables/disables its sublayers.
 #196 retrieve sublayers only if forced or enabled

SlidableWMSServiceLayerGroup should retrieve its sublayers only, if the
retrieval is forced, or the SlidableWMSServiceLayerGroup is enabled.
Also after the retrieval, the field refreshNeeded is set to false.
Merge branch 'feature/196-disable-loading-sildable-layer' into dev