Only add-layer event
Modules keep the information they need in response to the add-layer message. vissibility and transparency messages can be much lighter (just layer id and visibility value).
Only one visibility message (portal-layer-visibility has been removed while layer-visibility has been kept).
New layer tree with more use cases (placeholder and portalLayer with mapLayers containing legends, etc.)
Revert "Fixes #43"

This reverts commit 828c55b603c33e9e7e3edd4ebb2af107220f9468.

Should be done in "develop". Sorry.
Merge branch 'add-layer-events' into develop