Removing parameter handling from ObjectReader.

We want to ditch this class in the future anyways, and the current approach did not handle context varying fileNames, nor was it threadsafe.
SceneReaderPathPreview supports Alembic and most IECore.Reader files.

Also added OpenGLAttributes node to force GL points and curves.
SceneReaderPathPreview now supports SequencePaths with valid extensions.

It tries to set the frame range based on the FrameList of the FileSequence.
Focusing the viewer when the new plug is connected.
Replacing ReaderPathPreview with ImageReaderPathPreview.

Since most readers are now handled by the SceneReaderPathPreview, we don't need a second preview mode for them.
PathPreviewWidget now respects registration order.

This generally results in PathPreviewers being ordered from least to most specific.
Exposing an ObjectPreview node.

This simplifies other code which might want to mimic the preview behaviour (as with the _Camera node)
Tidying up SceneReaderPathPreview

Removed unused member variable, unnecessary nodule and widgets, fixed ext for SequencePaths, and simplified try/except block
Merge pull request #781 from andrewkaufman/ptcViewer

PathPreview improvements