TPSlider: Fix slider arrows

[imranhsayed]

• The disabled attribute was not working in the case of the nested slider because since querySelector was used it would find the first instance of the arrow which was from the inner slider instead of parent slider arrow.
• This PR fixes that issue by checking if the parent tp-slider of the arrow element is same as this. If it is then that means we have found the correct arrow element.
• we cannot use CSS for this because the user can keep the arrows anywhere they want.