Add support for creating promises from Qt signals (#25)

Introduce a new `QtPromise::connect()` helper that allows to create a promise resolved from a single signal and optionally, rejected by another one (from a different object or not). The promise type is determined by the type of the first signal argument (other arguments are currently ignored). A `QPromise<void>` is returned if the resolve signal doesn't provide any argument.

If the rejection is emitted before the promise is resolved, the promise will be rejected with the value of the first argument (other arguments being ignored). If the rejection signal doesn't provide any argument, the promise will be rejected with `QPromiseUndefinedException` if the signal is emitted. Additionally, the promise will be automatically rejected with `QPromiseContextException` if the source object is destroyed before the promise is resolved.

tests/auto/qtpromise/exceptions/tst_exceptions.cpp

@@ -15,6 +15,7 @@ class tst_exceptions : public QObject
 
 private Q_SLOTS:
     void canceled();
+    void context();
     void timeout();
     void undefined();
 
@@ -41,6 +42,11 @@ void tst_exceptions::canceled()
     verify<QPromiseCanceledException>();
 }
 
+void tst_exceptions::context()
+{
+    verify<QPromiseContextException>();
+}
+
 void tst_exceptions::timeout()
 {
     verify<QPromiseTimeoutException>();