Make Timeline grid snap work for all of the extra handles that some gadgets get when they're on a Timeline

At present, Timeline grid snap only works for a gadget's left edge, its right edge, and the whole gadget.

But those aren't the only things that we need to move on a Timeline. Some gadgets get extra handles when they're on a Timeline - for example, a Keyframe has a fade-in handle and a fade-out handle, and a Text Displayer has a text animation end handle.

Adjusting those extra handles is always freehand even when Timeline grid snap is on. It would be nice if they snapped to frame boundaries or second boundaries.