Added a helper function to validate an id and set/get a member from an id map.
Timer::hasBeen is now Timer::has_been to be consistent with other class interfaces.
Added an _id literal suffix.

It can be used to easily create a request/response id without weird
casts.
Added some support code to the client plugin.

The still starts the engine at 1000x1000 resolution then quits after one
second.