#191 avoid NPE in toElement()
 #191 made one constructor deprecated
 #191 read/write layer keywords

Write the layer keywords in toElement() and read them in
evaluateElementKeywords().
 #191 repaired constructor which receives an Element

Initialize a timer.
All read layers are added.
Read the keywords.
 #191 read/write the slider position

Write the slider position in toElement() and read it again in the
constructor which receives a Element. Afterwards it is set in internal
frame.
Merge branch 'bug/191-save-SlidableWMSServiceLayerGroup' into dev