Set focus to slide link upon mouse click on prev/next buttons

This is mainly an ergonomy/look & feel problem:

Now that the focus on prev/next buttons is handled by css, clicking one of these leave it highlighted even if the mouse cursor is moved away, because a click event also sets the focus.

This attached patch for branch 7.x.1.x detects that a click event really comes from the mouse (i.e.:not generated by keyboard) and in this case, moves the focus to the visible slide's anchor.

The mouse/keyboard detection is based on the time elapsed between the last mouse button change state and the click event (< 0.1 sec --> mouse click).

Thanks for considering it.