update: rewrite of layer dialog
new: allows multiple selection of layers in the dialog
new: move up, move down, toggle visibility, and delete on multiple layers
new: merge from an arbitrary layer into another layer, not only from the first into the second
new: new action for merging of the currently selected primitives on an arbitrary layer
new: make "active" layer explicit (special icon); activating a layer automatically moves it in the first position
refactoring: public fields 'name' and 'visible' on Layer are @deprecated. Use the setter/getters instead, Layer now emits PropertyChangeEvents if name or visibility are changed.