swap enhanced watchlist/RC arrow images for prettier ones

Replaced old blue ones with slick grey ones, taken from the Vector
skin (based on arrow-down-icon.png in skins/vector/images), but
adjusted to 12x12px (to keep the same dimensions as old icons for all
sorts of compatibility).

Also change the CSS positioning in changes list, to fit better with
the baseline of the text.

This arrow look is already used for the dropdown menu in the Vector
skin, as well as for collapsible navigation menu in the Vector
extension and in the editor interface in the WikiEditor extension.

This fixes bug 44853.

Bug: 44853
Change-Id: I36a5c2f856fefe222f73d41eb2a547e3c1822092
Merge "swap enhanced watchlist/RC arrow images for prettier ones"