UI plugin support

Add support to enable plugins to:

• add new workspaces

• add buttons to existing workspaces and workspace tabs

• add buttons to visit editor tabs

• register custom editors and viewers

Implementations will be limited to Echo 2 components.

This will require:

• the current user session to be exposed to plugins, via a Context object.
  This will provide access to the current practice, location, customer, patient, and clinician

• interfaces to be provided for UI components and be re-implemented in terms of the openvpms-framework-api

• echo2 and echopointng jars to repackaged as OSGi bundles