Let kdevelop-pg generate namespaces for the parser, the ast nodes,
the visitors and the user-defined namespaces. Basically, everything
except the user-defined code insertions from the start and the end
of the grammar file is namespaced now.

That has several positive aspects:
- be good neighbors with other kdev-pg based parsers
(two clashing expression_ast structs, anyone?)
- the parser class is finally named as such: class <language>::parser;
- better readability for most stuff

Drawback: You have to port some code,
how much it is depends on the amount of custom code extensions.
(As usual, kdev-pg's example grammars will be updated right away.)