GCS: Crash on closing

[tracernz]

I can reproduce by:

• having no boards connected to PC,
• running GCS,
• switch to Flight Data tab,
• switch to Welcome tab,
• close GCS

0   QWindow::parent() const         0x7ffff691e860  
1   QWindowContainer::parentWasRaised(QWidget *)            0x7ffff71f16f0  
2   ??          0x7ffff71f162e  
3   ??          0x7ffff71f162e  
4   ??          0x7ffff71f162e  
5   ??          0x7ffff71f162e  
6   ??          0x7ffff71f162e  
7   ??          0x7ffff71f162e  
8   ??          0x7ffff71f162e  
9   ??          0x7ffff71f162e  
10  ??          0x7ffff71f162e  
11  QWidget::raise()            0x7ffff71c8498  
12  QStackedLayout::setCurrentIndex(int)            0x7ffff71b7f65  
13  QStackedLayout::takeAt(int)         0x7ffff71b8924  
14  QLayout::removeWidget(QWidget *)            0x7ffff71b3aef  
15  Core::ModeManager::aboutToRemoveObject  modemanager.cpp 217 0x7fffc2592ca3  
16  Core::ModeManager::qt_static_metacall   moc_modemanager.cpp 136 0x7fffc25ddb92  
17  QMetaObject::activate(QObject *, int, int, void * *)            0x7ffff604c36e  
18  ExtensionSystem::PluginManager::aboutToRemoveObject moc_pluginmanager.cpp   183 0x7ffff78f5dc5  
19  ExtensionSystem::Internal::PluginManagerPrivate::removeObject   pluginmanager.cpp   569 0x7ffff78e7943  
... <More>              

Everything above 15 needs Qt library debug symbols which I don't have (yet). The object being removed is the "Welcome" tab (index = 0), and something downstream of the m_modeStack->removeTab(index); causes the crash. This may be deep in QML.

[tracernz]

Removing the PFD from the Flight Data tab stops the crash so this is almost certainly a case of something QML related being released twice.

[PTDreamer]

https://github.com/TauLabs/TauLabs/pull/2101