[Development] QProperty and library coding guide

[Ulf Hermann]

>> I've added a section to this one. Feel free to amend. However, newly
>> designing APIs with QProperty is a different thing than converting old
>> APIs in a compatible way.
> 
> The complexity behind the scenes for us to support it is our problem, users
> shouldn't care.
> 
> They will only care that the code they write is the same, regardless of
> whether it's an old or a new class.

Indeed the users shouldn't need to care. And they shouldn't use change 
signals. Rather, they should use subscribe(), and only if they cannot 
express the same thing using a binding. This way we can get rid of the 
notified properties in the long run.

So, new, QProperty-based, properties should not have change signals. 
That is something users may notice, and something that needs to be added 
to the API design principles.

best,
Ulf