Add ECM layers to deconflict entity creation and maintenance.

[nkoenig]

Desired behavior

The ECM maintains a set of IDs, one for each entity. This works fine when running simulation. Problems arise in the following cases:

1. When playing back a log file and a plugin wants to insert a new entity, such as a light. The new light entity can be overwritten by the ECM because the light's ID is in conflict with an ID from the log file. A patch to support this was added that allows setting an entity creation offset. See also Record entity count in a log file. #347.
2. A GUI model editor will want to modify its ECM without impacting ECM updates/changes from the server.

I imagine there might be other benefits to having layers, or a similar concept. For example, we could turn layers on/off to support different visualizations.