Add multithreading support

Make QPromise thread safe but also ensure that continuation lambdas (then/fail/finally) are called in the thread of the promise instance they are attached to.

tests/auto/benchmark/benchmark.pro

@@ -1,4 +1,3 @@
-QT += concurrent
 TARGET = tst_benchmark
 SOURCES += $$PWD/tst_benchmark.cpp
 

tests/auto/benchmark/tst_benchmark.cpp

@@ -2,7 +2,6 @@
 #include <QtPromise>
 
 // Qt
-#include <QtConcurrent>
 #include <QtTest>
 
 using namespace QtPromise;
@@ -11,11 +10,11 @@ class tst_benchmark: public QObject
 {
     Q_OBJECT
 
+private Q_SLOTS:
     void valueResolve();
     void valueReject();
     void valueThen();
     void errorReject();
-private Q_SLOTS:
     void errorThen();
 
 }; // class tst_benchmark