Add Layout Support for Pyface

Currently all usage of layouts in Pyface is ad-hoc toolkit-based usage for a handful of widgets (eg. some of the panel widgets). It would be desirable to introduce the concept of generic layout management to Pyface:

- to allow standardization of composition into larger UIs
- to allow creation of standard widgets that are composed of other lower-level widgets without needing platform-specific code
- to potentially create a framework that TraitsUI can use for building things