[Qt5-feedback] Improved QFuture

[Iván Todorovich]

QFuture should be improved to support custom subclasses. Nowadays
asynchronous calls are *very* important, and it should be easier to
make them. QFuture should be the standard result of every async method
and, in time, Qt should make use of that standard, for instance in
async http request (QNAM), QtMobility, sockets, an ideal QtSQL with
async support, etc.