Fix crash-on-exit in TimerModel

It was possible that signal_end_callback was called after TimerModel has
already been destructed.

Valgrind backtrace:
==3237== Invalid read of size 8
==3237==    at 0x2C2E1EB5: GammaRay::TimerModel::rowFor(QTimer*)
(timermodel.cpp:160)
==3237==    by 0x2C2E226F:
GammaRay::TimerModel::postSignalActivate(QTimer*) (timermodel.cpp:196)
==3237==    by 0x2C2E143F: signal_end_callback(QObject*, int)
(timermodel.cpp:74)
==3237==    by 0x5BC6AE7: QMetaObject::activate(QObject*, QMetaObject
const*, int, void**) (qobject.cpp:3603)
==3237==    by 0x5C282D2: QTimer::timeout() (moc_qtimer.cpp:148)
==3237==    by 0x5BCFCFA: QTimer::timerEvent(QTimerEvent*)
(qtimer.cpp:280)
==3237==    by 0x5BC104F: QObject::event(QEvent*) (qobject.cpp:1156)
==3237==    by 0x68F62EF: QApplicationPrivate::notify_helper(QObject*,
QEvent*) (qapplication.cpp:4562)
==3237==    by 0x68F3662: QApplication::notify(QObject*, QEvent*)
(qapplication.cpp:3944)
==3237==    by 0x80B31F5: KApplication::notify(QObject*, QEvent*)
(kapplication.cpp:311)
==3237==    by 0x5BA7A6B: QCoreApplication::notifyInternal(QObject*,
QEvent*) (qcoreapplication.cpp:946)
==3237==    by 0x5BAB6E4: QCoreApplication::sendEvent(QObject*, QEvent*)
(in /home/krf/devel/build/qt4.8-shared-debug/lib/libQtCore.so.4.8.4)
==3237==  Address 0xf78f620 is 0 bytes inside a block of size 40 free'd
==3237==    at 0x4C2A44B: operator delete(void*) (in
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3237==    by 0x2C2E4C01: GammaRay::TimerModel::~TimerModel()
(timermodel.h:38)
==3237==    by 0x5BC2443: QObjectPrivate::deleteChildren()
(qobject.cpp:1907)
==3237==    by 0x694A49A: QWidget::~QWidget() (qwidget.cpp:1681)
==3237==    by 0x2C2E1004: GammaRay::TimerTop::~TimerTop() (in
/home/krf/devel/build/kdab/GammaRay/lib/plugins/gammaray/gammaray_timertop_plugin.so)
==3237==    by 0x2C2E103F: GammaRay::TimerTop::~TimerTop()
(timertop.h:37)
==3237==    by 0x5BC2443: QObjectPrivate::deleteChildren()
(qobject.cpp:1907)
==3237==    by 0x694A49A: QWidget::~QWidget() (qwidget.cpp:1681)
==3237==    by 0x6E611FD: QFrame::~QFrame() (qframe.cpp:240)
==3237==    by 0x6EDF4EF: QStackedWidget::~QStackedWidget()
(qstackedwidget.cpp:194)
==3237==    by 0x6EDF527: QStackedWidget::~QStackedWidget()
(qstackedwidget.cpp:196)
==3237==    by 0x5BC2443: QObjectPrivate::deleteChildren()
(qobject.cpp:1907)