Selectable: fix mousedown on hierarchical selectable items.

Fixes a problem with selecting parents in selectable hierarchies.  Using .parents().addBack() resulted in an array of elements where the target element was added to the end.  Clicking directly on a child selectee would result in a parent being selected.

There are two alternatives on separate branches:

git remote add epowers https://github.com/epowers/jquery-ui.git

git checkout epowers/selectable_fix_alt_1
git checkout epowers/selectable_fix_1

I prefer the alt fix as it should perform better and have made that my master.  As well, master continues to get changes, if you will consider the next patch as well, adding a border tolerance option for lasso.  Basically, when attempting to lasso select child selectees in a parent selectee, the parent is always selected, even if the lasso does not touch the boundaries of the parent.  The new tolerance option eliminates selecting an element if the lasso is entirely within the bounds of the element.  The disadvantages of this option is that possibly some touch devices where an accidental lasso select within an element would have previously selected that element, in this border mode, will not.