Move command handling into queue

[ProGTX]

Get rid of the command_group class.
Already started with the introduction of the handler class: efeb82e

[ProGTX]

Don't really need to get rid of the command_group class - for now I've moved it into namespace detail (374b6d6), did some trimming (09a3dec), and used it to provide a local compilation environment (f3a4e4f, not done yet).

But yes, in the end, the queue should be the only one to handle commands. But this can now be done by using local compilation environments (local command groups) and later joining them. This should allow for better optimizations, as all commands can be viewed at once.

[ProGTX]

There were some problems with the local compilation environment, had to be removed in 10167ad.

[ProGTX]

Basically done in 1ec0a7b, but needs #13. Closing.