Protect PostIt deletion by SolarMutex.

I was getting crashes during editing with notes; unfortunately I am unable to
reproduce it reliably, so I am not 100% sure this is the right fix - but I
hope so.

This is what valgrind was suggesting:

==21766== Invalid read of size 8
==21766==    at 0x285AF9C7: std::__debug::list<SwSidebarItem*, std::allocator<SwSidebarItem*> >::remove(SwSidebarItem* const&) (list:569)
==21766==    by 0x285A2BFC: SwPostItMgr::RemoveItem(SfxBroadcaster*) (PostItMgr.cxx:241)
==21766==    by 0x285A3241: SwPostItMgr::Notify(SfxBroadcaster&, SfxHint const&) (PostItMgr.cxx:346)
==21766==    by 0x84AFB1E: SfxBroadcaster::Broadcast(SfxHint const&) (brdcst.cxx:46)
...
==21766==    by 0xA5A3C74: Application::Yield() (svapp.cxx:393)
==21766==    by 0xA5A3C20: Application::Execute() (svapp.cxx:342)
==21766==    by 0x50EA0F4: desktop::Desktop::Main() (app.cxx:1678)
...
==21766==  Address 0x2e2824f0 is 16 bytes inside a block of size 24 free'd
==21766==    at 0x4C2AA9C: operator delete(void*) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==21766==    by 0x285B4B85: __gnu_cxx::new_allocator<std::__cxx1998::_List_node<SwSidebarItem*> >::deallocate(std::__cxx1998::_List_node<SwSidebarItem*>*, unsigned long) (new_allocator.h:100)
==21766==    by 0x285B40C1: std::__cxx1998::_List_base<SwSidebarItem*, std::allocator<SwSidebarItem*> >::_M_put_node(std::__cxx1998::_List_node<SwSidebarItem*>*) (stl_list.h:339)
==21766==    by 0x285B43BB: std::__cxx1998::list<SwSidebarItem*, std::allocator<SwSidebarItem*> >::_M_erase(std::__cxx1998::_List_iterator<SwSidebarItem*>) (stl_list.h:1549)
==21766==    by 0x285B346E: std::__cxx1998::list<SwSidebarItem*, std::allocator<SwSidebarItem*> >::erase(std::__cxx1998::_List_iterator<SwSidebarItem*>) (list.tcc:113)
==21766==    by 0x285B18EB: std::__debug::list<SwSidebarItem*, std::allocator<SwSidebarItem*> >::_M_erase(std::__cxx1998::_List_iterator<SwSidebarItem*>) (list:423)
==21766==    by 0x285AF9E6: std::__debug::list<SwSidebarItem*, std::allocator<SwSidebarItem*> >::remove(SwSidebarItem* const&) (list:570)
==21766==    by 0x285A2BFC: SwPostItMgr::RemoveItem(SfxBroadcaster*) (PostItMgr.cxx:241)
==21766==    by 0x285A3241: SwPostItMgr::Notify(SfxBroadcaster&, SfxHint const&) (PostItMgr.cxx:346)
==21766==    by 0x84AFB1E: SfxBroadcaster::Broadcast(SfxHint const&) (brdcst.cxx:46)
...
==21766==    by 0xA5A3C74: Application::Yield() (svapp.cxx:393)
==21766==    by 0xA5A3C20: Application::Execute() (svapp.cxx:342)
==21766==

Change-Id: I0ed3e09338810200766bd10203d9fcac26e822f6