Group related concepts
Remove new from Entities trait

It complicates initialization and alternative implementations for no
apparent benefit.
Add control that provides deferred actions

The control will provide an interface for adding, importing or removing
entities. Those actions will be applied later to not interfere with any
ongoing iteration over the components.
Add Control::import
Add Control::remove
Only generate even id numbers

This enables Control to generate its own id numbers that it can return
from add.
Return id from Control::add
Remove unused code
Remove unnecessary pubs
Clear added and imported entities after apply
Clear removed entities after apply