":1" and ":mixed" are available as attributes on accessCondition, but using either of these causes an error

[mdaines]

(Related to #119, #122)

If a crosswalk is created as follows, with a ":1" or ":mixed" attribute on an element and fixed text wired up to the attribute, an error occurs when running the crosswalk.

!ENTRY org.eclipse.core.jobs 4 2 2013-01-28 10:33:56.199
!MESSAGE An internal error occurred during: "Running panopto.crosswalk".
!STACK 0
java.lang.IllegalArgumentException: The datatype 'EFeatureMapEntry' is not a valid classifier
    at org.eclipse.emf.ecore.impl.EcoreFactoryImpl.createFromString(EcoreFactoryImpl.java:160)
    at org.eclipse.emf.ecore.util.EcoreUtil.createFromString(EcoreUtil.java:3341)
    at crosswalk.impl.MappedAttributeImpl.updateRecord(MappedAttributeImpl.java:529)
    at crosswalk.impl.MappedElementImpl.updateRecord(MappedElementImpl.java:289)
    at unc.lib.cdr.workbench.xwalk.CrosswalkJob.processRecord(CrosswalkJob.java:283)
    at unc.lib.cdr.workbench.xwalk.CrosswalkJob.run(CrosswalkJob.java:161)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)

[mdaines]

Possible ways to address this:

• These attributes, or this particular combination, could be filtered out in MappedModelUtil.getAttributes or PickMappedFeatureCommand (to avoid displaying attributes named :1 and :mixed)
• MappedAttributeImpl#updateRecord could handle setting these attributes differently (detect that createFromString isn't applicable, don't use eSet?)