ctools_context_optional regression with public $required property

#2829295: Improve test coverage and function docs: context introduced a regression. The class ctools_context_optional is missing the required property override to FALSE from the parent ctools_context_required class.

This caused page elements on context admin pages to completely disappear.

Solution: define class property default values on the property itself where possible. That way subclasses can easily override them and don't need to copy the constructor.