Initial support for toggling visibility

[atc0005]

Changes

• add Element.Visible field
  • to control visibility of Elements
  • if not specified, this value defaults to true by omitting the field from the generated JSON payload
• add TargetElement type
• add Action.TargetElement field
  • to specify collection of TargetElement values
  • used to specify target Element values whose visibility state should be controlled by the Element whose Action the TargetElement field is associated with
• add ISelectAction.TargetElements field
  • same purpose as Action.TargetElement field
• add missing validation for Element.SelectAction field for Element of ColumnSet type
• add missing validation for Element.SelectAction field for Element of Container type
• add Column.AddSelectAction method
  • accepts Action or ISelectAction value
• update ISelectAction.Validate method
  • add missing validation for ISelectAction.Fallback field
  • add validation for ISelectAction.TargetElements field for Action.ToggleVisibility type
• update Action.Validate method
  • convert validation logic to use validator package
• add Action.AddTargetElement method
  • accepts multiple Element values
  • allows explicitly setting target Element visibility
• add Action.AddVisibleTargetElement method
  • convienence method to explicitly enable visibility for target Element values
• add Action.AddHiddenTargetElement method
  • convienence method to explicitly disable visibility for target Element values
• add Action.AddTargetElementID method
  • accepts multiple Element ID values
  • allows explicitly setting target Element visibility
• add new helper "constructor" functions
  • NewHiddenerContainer
  • NewColumn
  • NewColumnSet
  • NewHiddenTextBlock
  • NewActionToggleVisibility
• update Container.AddAction
  • add note to doc comments directing reader to Container.AddSelectAction for adding an Action to be used when the Container is tapped or selected
• add Container.AddSelectAction method
  • accepts Action or ISelectAction value
• add examples
  • add toggle-visibility-column-action
  • add toggle-visibility-container-action
  • add toggle-visibility-multiple-buttons
  • add toggle-visibility-single-button

References

• fixes Support for ToggleVisibility #243
• fixes How to handle ToggleVisibility for table/columns #239