prop: Add property argument
prop: scale can be NULL, TRUE, FALSE, or string
More uses of string scale
prop.format: include property and properly print no scale
Update props() to use new prop objects
props(): handle events
Rename 'propset' to 'prop_event'
Update prop tests
props(): evaluate arguments in correct environment
Update tests for new prop syntax
Update test specs
Rename prop_name to prop_label
Rename propset to event in docs
Remove unused function
Simplify code for registering scales from props
Remove property from scales specs
Use correct variable
Remove valid_scale and valid_scale_types
Use S3 dispatch for creating prop objects
More S3 conversion of prop functions