Fix layers control on WinPhone8/IE10 Touch

[danzel]

Fixes #1539

[mourner]

Thanks for the fix Dave! Can you explain the fix btw? I'm not sure what's happening here :)

[danzel]

IE10 generates a mouseover event when you put your finger down and a mouseout event when you release, this is what was breaking the layers control before.

This clued me in on on the fix:
http://www.cssplay.co.uk/menus/cssplay-droplist-menu-ie10-touch-screen.html
http://wordpress.org/support/topic/ie10-ie8-on-touch-devices

Setting aria-haspopup somehow makes the events work right, I'm not totally sure what it changes, I think it makes it stop firing the mouseover event when you release your initial touch (I stopped investigating when it worked).

[mourner]

Allright. :) I think it's worth adding a comment to the fix then, as it's not obvious (you can commit this directly without a pull).