mapfishapp - layer menu item to switch between baselayer and overlay

With this PR, a new layer menu item is added, which allows to switch back and forth between baselayer and overlay.

This impacts:
 - the layer order (baselayers at the bottom, overlays at the top),
 - the layer transitionEffect (backbuffers shown after animated zoom for baselayers, hidden for overlays),
 - the layer node ui color in the tree panel: baselayers are now painted with #f2f0f5 (overridable via CSS).