This specifies some of the detail within the Invokers proposal (whatwg#9625).

This introduces 2 new IDLs:

- InvokeEvent: A new event that has `action` and `invoker`.
- InvokeElement: A mixin applied to Buttons to add
  `invokeTargetElement`/`invokeAction`

It also adds the `invoketarget` & `invokeaction` attributes which are
reflected to the IDL properties.

It also adds the steps for invocation: button activation checks if the
node has an invokeTarget and dispatches an InvokeEvent if so.

Things NOT covered in this commit that are included in the proposal (whatwg#9625):

- Default per element behaviours (invocation action algorithms), this
  will be dealt with in subsequent individual commits.

- `interestaction` and `interesttarget`.

This specifies what buttons with an invoketarget pointing to an element
with `popover` should do, based on the Invokers proposal (whatwg#9625).

This introduces new specifications just within the "invoke target attribute activation behavior" algorithm such that:

 - If an `invoketarget` points to an element with `popover`
    - If the `invokeaction` is `auto` or `togglePopover`, try to toggle the popover
    - If the `invokeaction` is `hidePopover`, try to hide the popover
    - If the `invokeaction` is `showPopover`, try to show the popover

If the `invokeaction` is none of the above, then it will fall through
the logic and be passed over to the "invocation action algorithm" for
the invokee element, if present.

Things NOT covered in this commit that are included in the proposal (whatwg#9625):

- Default per element behaviours (invocation action algorithms) beyond
  popovers. These will be dealt with in subsequent commits adding each
  elements "invocation action algorithm".

- `interestaction` and `interesttarget`.

This specifies what buttons with an invoketarget pointing to a dialog element
should do, based on the Invokers proposal (whatwg#9625).

This introduces new specifications within the HTMLDialogElement, giving it an "associated invocation action algorithm" such that:

 - If an `invoketarget` points to an HTMLDialogElement element
    - If the `invokeaction` is `auto` or `close`, and the Dialog is
      open, then try to close the dialog.
    - If the `invokeaction` is `auto` or `showmodal` and the Dialog is
      not open, then try to open the dialog.

If the `invokeaction` is none of the above, then it will do nothing.

Things NOT covered in this commit that are included in the proposal (whatwg#9625):

- Other per element behaviours (invocation action algorithms) beyond
  HTMLDialogElement. These will be dealt with in subsequent commits
  adding each elements "invocation action algorithm".

- `interestaction` and `interesttarget`.

This assists developers (and to a degree implementers) in providing a
quick reference from which to see the action hints that an invokeaction
may supply in order to have an effect on given elements.

See openui/open-ui#900

This also makes distinct the "valid custom invokeaction" algorithm

This allows us to remove the checks in invoke target attribute activation behavior

…viour

…nvoketarget.

…aviour

…related steps