Support target element reference

[yinonov]

any hints on strategy for adding support to JS target element reference?
openui/open-ui#651 (comment)

[keithamus]

• popoverToggleTargetElement, popoverShowTargetElement, popoverHideTargetElement will all be properties that will be exposed on HTMLElement
• We'll need a weakmap for each that stores has the key of the element (this) and the value that it can be set to (the target element).
• The setter function should set the weakmap to the value given
• The getter function will need to run through some steps (reading from the spec):
  • If the el.disabled return null
  • If el.form && isSubmitButton(el) return null
  • If the WeakMap value is not falsey
    • If value's .popover value is 'auto' or 'manual' returns the value
    • Else return null
  • If the WeakMap value is empty...
    • Call getAttribute() with the respective attribute
    • If the attribute is falsey, return null
    • If the attribute is a string, call get el = document.getElementById(attr)
      • If value's .popover value is 'auto' or 'manual' returns the value
      • Else return null

[yinonov]

Many thanks @keithamus. started a draft #70. will ping once I feel it's ready for review