Attribute

• Allows a stack to be selected as an attribute value.

Helper

• Allows a "select stack" form field to be used in block forms.  Works similar to how the "page select" field works.
• Allows a "select multiple stack" form field to be used in block forms.  Works similar to the "slideshow" add image field.

Tools

• Attribute and helpers will load a slightly modified version of the "Add stack to area" pop-up that is native to the concrete5 core, meaning there's nothing for new users to learn.

Special thanks to Michael Krasnow for his work in the page_select_attribute and Andrew Embler for his work with the stack_select_popup which were both referenced heavily when creating this package.