kdwsdl2cpp: Constructor of generated client should be marked explicit

[mg34343]

kdwsdl2cpp generates constructors like this:

ClientName( QObject* _parent = nullptr );

The constructor should be marked as explicit so it will not be considered as a converting constructor:

explicit ClientName( QObject* _parent = nullptr );

Motivation:
Just had to debug a problem in my code where I accidentally called a function that expected a ClientName& with a ClientName* instead, which resulted in the function being called with a new ClientName that had the original set as parent. With the change these sort of mistakes would result in a compiler error instead. Qt has all constructors with a single QObject* parameter marked as explicit for this reason.

[dfaure-kdab]

@martonmiklos interested in adding this to libkode?

[martonmiklos]

@dfaure-kdab
Absolutely, I will come up with a PR to support explicit specifier for functions.