vtkPVGeneralSettings and thread safety

vtkPVGeneralSettings and others are not currently used in a thread-safe way. Here're a few challenges:

• We made settings a proxy. Having it a proxy makes it possible to easily show panels to modify them -- Settings Dialog and also link properties with others proxies.
• However, since settings becomes a proxy, accessing it via proxy API means we'll need to access them from the current session. This is silly since many settings are not session specific e.g. GUIFontSize. So for those, we ended up directly accessing the single VTK object these settings proxies refer to -- that's no longer thread safe.