vcl::EventPoster shall probably be covered by SolarMutex

...as most parts of VCL.  Ran across at least one case where a remote call to
framework::CloseDispatcher::release -> ~CloseDispatcher -> ~EventPoster ->
Application::RemoveUserEvent caused a crash.  As always with SolarMutex, keep
fingers crossed that this is about the right level to acquire it.

Change-Id: I8f4be7329adbf72355774fa5d3c472270da3ddd2