tdf#155794 winaccessibility: no SolarMutex in getAccObjectPtr()
MSAAServiceImpl::getAccObjectPtr() is called when processing
WM_GETOBJECT messages, and this can happen (at least when NVDA is
active) during processing SendMessages.
When loading a document on a non-main thread, WinSalFrame::SetTitle()
calls SetWindowTextW which is equivalent to SendMessage(WM_SETTEXT),
while holding SolarMutex, and if the main thread doesn't finish
processing it then that's a deadlock.
Introduce a new mutex in AccObjectWinManager and use it to guard the 2
members that getAccObjectPtr() reads, while keeping the rest of
winaccessibility with the SolarMutex, as the UNO services may be called
on any thread.
This fixes part of the problem, VCL also needs to stop using SolarMutex.
Change-Id: I6df5889fd76f59146b4b0b1e5f4513232f8ab867