Make qt wallet test compatible with qt4

Unlike Qt5, the Qt4 signals implementation doesn't allow a signal to be
directly connected to a c++ lambda expression. Work around this by defining a
Callback QObject with a virtual method that can forward calls to a closure.

The Qt4 error was reported by Patrick Strateman <patrick.strateman@gmail.com>
in https://github.com/bitcoin/bitcoin/pull/10039#issuecomment-289248763

src/Makefile.qt.include

@@ -122,6 +122,7 @@ QT_MOC_CPP = \
   qt/moc_bitcoinamountfield.cpp \
   qt/moc_bitcoingui.cpp \
   qt/moc_bitcoinunits.cpp \
+  qt/moc_callback.cpp \
   qt/moc_clientmodel.cpp \
   qt/moc_coincontroldialog.cpp \
   qt/moc_coincontroltreewidget.cpp \
@@ -167,6 +168,7 @@ BITCOIN_MM = \
 QT_MOC = \
   qt/bitcoin.moc \
   qt/bitcoinamountfield.moc \
+  qt/callback.moc \
   qt/intro.moc \
   qt/overviewpage.moc \
   qt/rpcconsole.moc
@@ -189,6 +191,7 @@ BITCOIN_QT_H = \
   qt/bitcoinamountfield.h \
   qt/bitcoingui.h \
   qt/bitcoinunits.h \
+  qt/callback.h \
   qt/clientmodel.h \
   qt/coincontroldialog.h \
   qt/coincontroltreewidget.h \