[FIXED][Issue 8] Two mouse clicks required for top level menu in Firefox, IE, Opera

Fixes
https://github.com/adobe-accessibility/Accessible-Mega-Menu/issues/8

_focusInHandler needs to track both mouseFocus and keyboardFocus. With
keyboard focus we wan the subsequent click event to open the menu, but
with mouse focus, when the menu is open, we want to click through the
link.