Added workaround for Qt OpenGL problem on OS X.

This would manifest itself as "invalid framebuffer" errors leaking into the IECoreGL and PyOpenGL code and being reported there, causing various forms of havoc.

Fixes #404.
Fixed "invalid drawable" error messages on OS X.

I had hoped that this fix would make the previous commit unnecessary, but testing shows that they address different problems and both are needed.

Fixes #396.
Fixed QEvent enum reference for PyQt.
Updated view app to display all relevant PathPreviews.

This makes it capable of showing more information than just the basic viewer it used before - IndexedIO and header previews for instance.
WIP