This allows users to style function names at their definitions.

Once set, the XML-style token "FunctionDefinition" will
highlight all function definitions: the style option is a
mixin to Function and Virtual Function.

TEST=Default themes and locally hacked themes that lack Function,
FunctionDefinition, Declaration-styling look as they did before
this patch.

Requires Clang.

Task-number: QTCREATORBUG-16625
Change-Id: I49d8e401211bdf28ff74699feac16fe98f6d64ce
Reviewed-by: Marco Bubke <marco.bubke@qt.io>