ui: slide{Up,Down,Left,Right} should slide as much as possible

Right now these four functions try to slide by the given delta value. If they cannot slide by that amount, they don't move and return false. Instead, they should try to slide as much as possible up to delta. This has two benefits: 1) often if each frame slides a bit, it can amount to enough further up the recursion stack, 2) frames will be able to slide flush with the side of the window.