Add an aria-haspopup attribute to the toggle button

I've created a MR adding the aria-haspopup attribute to the button. One detail i've noticed eventhough on the issue page here the MR is shown as green if you go over to the corresponding issue on the gitlab page you will notice there is an eslint error(s), but all of them seem unrelated to this minor change? that is way out of my comfortzone and level of expertise since i am not a developer. ;)

@rkoller Thank you for the issue and the MR! The The JS must refactored for ESLint - but this is another issue.

MR checked out an the aria-attribute appears in the markup.

Automatically closed - issue fixed for 2 weeks with no activity.