Support infrastructure to call properties (primarily setters) directly

Going further with QML compiler, we need a way to figure out how to directly call property functions, without involving the meta object system.

Consider an example:

Rectangle {
  width: 42
}

In QML compiler, property setting would be translated into something like:

this->setProperty("width", 42);

However, in this case we rely on the meta object system to figure out the setter of the property.

Likely, the direct call is much faster, so having

this->setWidth(42); // directly call C++ method of "this"
emit this->witdhChanged(); // optional?

would give better performance, practically allowing the auto-generated code to be as good as human-generated one in certain cases.

For now, looks like only setters are interesting (and signals?).