PathPreview improvements

This brings several improvements to various PathPreviewWidgets. The motivation was to get a nice preview of PTC file sequences. Note that the browser app doesn't actually import IECoreRI, so in order to preview PTCs, that would need to be done from an external config for now.

- Improvements to SceneReaderPathPreview
 - Support for Alembic and most IECore.Reader files (but not images)
 - Forces GL points and curves (rather than disks and ribbons)
 - Focuses the viewer when the file changes
 - Support for SequencePaths with valid extensions
  - Sets the timeline frame range based on the FrameList of the FileSequence
-  Replaced ReaderPathPreview with ImageReaderPathPreview
 - I didn't see a need for both the Preview and Scene tabs on the genuine 3d objects
- PathPreviewWidget now respects registration order
 - This generally results in PathPreviewers being ordered from least to most specific
- Fixed fileName update bug in ObjectReader node