Let server manipulate `Composition`s

• Remove saveLayout function, as now Blending Editor, should be responsible for that.
  Starcounter/Blending#471
• Drop support for compositionProvider.Composition, only compositionProvider.Composition$ is supported
• Element observes Polymer notifications for compositionProvider.Composition - to be able to react on server-side changes triggered by any means, not just composition editor custom element.
• Save updated compositionProvider's custom composition as stored one, even if it's the same as current temporary one, as after closing the editor changes were reverted not to the saved state, but to the state before editing was started Starcounter/Blending#460
• more verbose partial attribute deprecation warning - now it shows a reference to the problematic instance