[Development] FileRe: Move ctors for q_declare_shared types

[Daniel Teske]

Hi,

https://codereview.qt-project.org/#/c/120804/ and
https://codereview.qt-project.org/#/c/120771 do fix the move assignment 
operators in Qt, just like I wanted. 

I wonder why Marc Mutz didn't fell the need to update the mailing list on his 
changed opinion and this fix. 

Anyway, for those who don't want to read the full thread but want to know how 
write a move assignment operator. This advice by Howard E. Hinnant is good:

"In general a move assignment operator should:
- Destroy visible resources (though maybe save implementation detail 
resources).
- Move assign all bases and members.
- If the move assignment of bases and members didn't make the rhs resource-
less, then make it so."

In code this looks e.g. like this:
 QVector<T> &operator=(QVector<T> &&other) Q_DECL_NOTHROW
{ QVector moved(std::move(other)); swap(moved); return *this; }

Now, if and only if the destructor of the class has no side effects or those 
side effects are inconsequential, you might consider implementing move 
assignment as a swap between other and this.

But if you are holding a user defined type or if you are holding resources like 
file handlers or network sockets, you shouldn't implement move assignment via a 
simple swap between other and this.

As a counter example, std::string's move assignment operator is allowed to be 
implemented as a simple swap.

daniel