Improve signal signatures to avoid qOverloadOf in QtCore

Some classes are using overloads for signals. While this was perfectly fine in a Qt 4 context and the old connect syntax, this forces to users to qOverloadOf connect statements due to ambiguity. This results in hard to read code and could be optimized by more explicit signatures.
 

The idea is not to remove qOverloadOf completely, rather to reduce the need for it in the API itself for Qt 6.