more correct creation and edit times for entities

Cleanup of how EntityItem::_created (and friends) are set during initialization and from server updates that use EntityItem::setProperties().  Here's a summary of how it now works:

(1) EntityItem::_created is initialized to 0 in the ctors.  A value of 0 means "creation time is not yet known".  Specifically at that context we don't know if this EntityItem is going to be filled out with data from the EntityServer, or by data from a script creation event.

(2) In EntityItem::setProperties() the creation timestamp is ignored if EntityItem::_created already has a non-zero value.

(3) The path by which scripts create entities doesn't automatically set EntityItemProperties::_created to a useful value therefore we must manually set it after the entity is created.
This is one place where we set EntityItem::_created to a valid value: when unpacking from a network packet (maybe this also handles reading from file?  dunno).