Autogen: Add Keywords class for frequently used keyword strings
This adds the cmQtAutoGenGlobalInitializer::Keywords
class. It holds various preallocated std::string
instances for keywords that are frequently used in QtAutogen. The lifetime of the Keywords
class instance is bound to the lifetime of the cmQtAutoGenGlobalInitializer
instances which guarantees that the strings are only allocated in the context where they are needed. This is in contrast to global static const std::string
instances that persist the whole runtime of the program, even when they are not used.