Write a generic C++ code generator

The current way of generating C++ code in qmlcachegen and similar tools is brittle and error-prone. It relies on string concatenation and makes it hard to predict the generated code from reading the generator code. Furthermore, the generated code tends to be messy also hard to read. There should either be a template system or a structured code generator like the one we use for generating QML code in qmlplugindump.