Fix Invalid read of size 4 in KateApp::~KateApp()

The problem was, that QList<KateMainWindow *> m_mainWindows; was deleted first,
and ~KatePluginManager() accesses this list during its destruction (unloading of
plugins GUI).

==3956== Invalid read of size 4
==3956==    at 0x48D0FB: QListData::size() const (qlist.h:107)
==3956==    by 0x492A63: QList<KateMainWindow*>::size() const (qlist.h:160)
==3956==    by 0x4920E5: KateApp::mainWindowsCount() const (kateapp.cpp:368)
==3956==    by 0x4A9C5E: KatePluginManager::disablePluginGUI(KatePluginInfo*) (katepluginmanager.cpp:274)
==3956==    by 0x4A9772: KatePluginManager::unloadPlugin(KatePluginInfo*) (katepluginmanager.cpp:196)
==3956==    by 0x4A944C: KatePluginManager::unloadAllPlugins() (katepluginmanager.cpp:156)
==3956==    by 0x4A8805: KatePluginManager::~KatePluginManager() (katepluginmanager.cpp:52)
==3956==    by 0x4904C1: KateApp::~KateApp() (kateapp.cpp:71)
==3956==    by 0x48C79F: main (main.cpp:429)
==3956==  Address 0x1bb5612c is 12 bytes inside a block of size 32 free'd
==3956==    at 0x4C2A37C: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==3956==    by 0xAE80408: QListData::dispose(QListData::Data*) (qlist.cpp:166)
==3956==    by 0x49360C: QList<KateMainWindow*>::dealloc(QListData::Data*) (qlist.h:857)
==3956==    by 0x492C1F: QList<KateMainWindow*>::~QList() (qlist.h:817)
==3956==    by 0x49048D: KateApp::~KateApp() (kateapp.cpp:71)
==3956==    by 0x48C79F: main (main.cpp:429)
==3956==
==3956== Invalid read of size 4
==3956==    at 0x48D105: QListData::size() const (qlist.h:107)
==3956==    by 0x492A63: QList<KateMainWindow*>::size() const (qlist.h:160)
==3956==    by 0x4920E5: KateApp::mainWindowsCount() const (kateapp.cpp:368)
==3956==    by 0x4A9C5E: KatePluginManager::disablePluginGUI(KatePluginInfo*) (katepluginmanager.cpp:274)
==3956==    by 0x4A9772: KatePluginManager::unloadPlugin(KatePluginInfo*) (katepluginmanager.cpp:196)
==3956==    by 0x4A944C: KatePluginManager::unloadAllPlugins() (katepluginmanager.cpp:156)
==3956==    by 0x4A8805: KatePluginManager::~KatePluginManager() (katepluginmanager.cpp:52)
==3956==    by 0x4904C1: KateApp::~KateApp() (kateapp.cpp:71)
==3956==    by 0x48C79F: main (main.cpp:429)
==3956==  Address 0x1bb56128 is 8 bytes inside a block of size 32 free'd
==3956==    at 0x4C2A37C: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==3956==    by 0xAE80408: QListData::dispose(QListData::Data*) (qlist.cpp:166)
==3956==    by 0x49360C: QList<KateMainWindow*>::dealloc(QListData::Data*) (qlist.h:857)
==3956==    by 0x492C1F: QList<KateMainWindow*>::~QList() (qlist.h:817)
==3956==    by 0x49048D: KateApp::~KateApp() (kateapp.cpp:71)
==3956==    by 0x48C79F: main (main.cpp:429)