Cost tooltip hides properly when mouse leaves

Previously, the cost tooltip would remain active if the cursor hovered over it. This caused some minor annoyances in the Room screen, and some potentially fatal problems in combat (trying to eat food/meds while the cost tooltip from a rifle was still up...).

Ideally this could be fixed in CSS, but, from what I can tell, the [best solution](https://developer.mozilla.org/en-US/docs/Web/CSS/pointer-events) isn't [fully supported](http://caniuse.com/pointer-events) yet, and it's simple enough to do in jQuery.