Fix crash with script engine debugger in use

If parent is set on our custom QStyle instance, gammaray's QMainWindow
destructor is going to delete the style right away -
qscriptenginedebugger still seems to hold an internal reference to it
and crashes accessing it on destruction.

Backtrace (trimmed):
Thread 1 (Thread 0x7fd1ec642780 (LWP 15343)):
[KCrash Handler]
4  0x00000000000000e0 in ?? ()
5  0x00007fd1eafb53b5 in QStyleSheetStyle::pixelMetric (this=0x2d00a70,
m=QStyle::PM_DockWidgetTitleBarButtonMargin, opt=0x0, w=0x2d90ee0) at
styles/qstylesheetstyle.cpp:4790
6  0x00007fd1eb06943f in QDockWidgetTitleButton::sizeHint
(this=0x2d90ee0) at widgets/qdockwidget.cpp:138
7  0x00007fd1eb06acf6 in QDockWidgetLayout::minimumTitleWidth
(this=0x2d77b80) at widgets/qdockwidget.cpp:416
8  0x00007fd1eb06adf0 in QDockWidgetLayout::sizeFromContent
(this=0x2d77b80, content=..., floating=false) at
widgets/qdockwidget.cpp:274
9  0x00007fd1eb06b1af in QDockWidgetItem::maximumSize (this=0x3202c40)
at widgets/qdockwidget.cpp:558
10 0x00007fd1eb06eba8 in QDockAreaLayoutItem::maximumSize
(this=0x3215620) at widgets/qdockarealayout.cpp:154
11 0x00007fd1eb06ee5f in QDockAreaLayoutInfo::saveState
(this=0x30a2338, stream=...) at widgets/qdockarealayout.cpp:1828
12 0x00007fd1eb06f9b1 in QDockAreaLayout::saveState (this=0x30a22f8,
stream=...) at widgets/qdockarealayout.cpp:2306
13 0x00007fd1eb0989d0 in QMainWindowLayoutState::saveState
(this=0x30a2220, stream=...) at widgets/qmainwindowlayout.cpp:598
14 0x00007fd1eb096cb7 in QMainWindow::saveState (this=0x3081810,
version=0) at widgets/qmainwindow.cpp:1237
15 0x00007fd1c5046ea5 in
QScriptEngineDebuggerPrivate::~QScriptEngineDebuggerPrivate
(this=0x2ddf810, __in_chrg=<optimized out>) at
debugging/qscriptenginedebugger.cpp:314
16 0x00007fd1c50470b9 in
QScriptEngineDebuggerPrivate::~QScriptEngineDebuggerPrivate
(this=0x2ddf810, __in_chrg=<optimized out>) at
debugging/qscriptenginedebugger.cpp:320
17 0x00007fd1eb903bd0 in cleanup (pointer=<optimized out>) at
../../include/QtCore/../../src/corelib/tools/qscopedpointer.h:62
18 ~QScopedPointer (this=0x2ddf8e8, __in_chrg=<optimized out>) at
../../include/QtCore/../../src/corelib/tools/qscopedpointer.h:100
19 QObject::~QObject (this=0x2ddf8e0, __in_chrg=<optimized out>) at
kernel/qobject.cpp:817
20 0x00007fd1c5046bb9 in QScriptEngineDebugger::~QScriptEngineDebugger
(this=0x2ddf8e0, __in_chrg=<optimized out>) at
debugging/qscriptenginedebugger.cpp:366
21 0x00007fd1eb8fd935 in QObjectPrivate::deleteChildren
(this=0x3092650) at kernel/qobject.cpp:1908
22 0x00007fd1eacb8740 in QWidget::~QWidget (this=0x2924f70,
_in_chrg=<optimized out>) at kernel/qwidget.cpp:1676
23 0x00007fd1c52c849d in
GammaRay::ScriptEngineDebugger::~ScriptEngineDebugger (this=0x2924f70,
__in_chrg=<optimized out>) at
/home/krf/devel/src/kdab/GammaRay/tools/scriptenginedebugger/scriptenginedebugger.cpp:65
24 0x00007fd1c52c850c in
GammaRay::ScriptEngineDebugger::~ScriptEngineDebugger (this=0x2924f70,
__in_chrg=<optimized out>) at
/home/krf/devel/src/kdab/GammaRay/tools/scriptenginedebugger/scriptenginedebugger.cpp:68
25 0x00007fd1eb8fd935 in QObjectPrivate::deleteChildren
(this=0x2d36a60) at kernel/qobject.cpp:1908
26 0x00007fd1eacb8740 in QWidget::~QWidget (this=0x2d36a30,
__in_chrg=<optimized out>) at kernel/qwidget.cpp:1676
27 0x00007fd1eb0d94b9 in QStackedWidget::~QStackedWidget
(this=0x2d36a30, __in_chrg=<optimized out>) at
widgets/qstackedwidget.cpp:193
28 0x00007fd1eb8fd935 in QObjectPrivate::deleteChildren
(this=0x2d00ac0) at kernel/qobject.cpp:1908
29 0x00007fd1eacb8740 in QWidget::~QWidget (this=0x25dcfc0,
__in_chrg=<optimized out>) at kernel/qwidget.cpp:1676
30 0x00007fd1eacb8a19 in QWidget::~QWidget (this=0x25dcfc0,
__in_chrg=<optimized out>) at kernel/qwidget.cpp:1700
31 0x00007fd1eb8fd935 in QObjectPrivate::deleteChildren
(this=0x2cf2b10) at kernel/qobject.cpp:1908
32 0x00007fd1eacb8740 in QWidget::~QWidget (this=0x2bb8ff0,
__in_chrg=<optimized out>) at kernel/qwidget.cpp:1676
33 0x00007fd1ec3160f8 in GammaRay::MainWindow::~MainWindow
(this=0x2bb8ff0, __in_chrg=<optimized out>) at
/home/krf/devel/src/kdab/GammaRay/core/mainwindow.h:34
34 0x00007fd1ec316130 in GammaRay::MainWindow::~MainWindow
(this=0x2bb8ff0, __in_chrg=<optimized out>) at
/home/krf/devel/src/kdab/GammaRay/core/mainwindow.h:34