ui: added some placeholder handling and changed search node ui

[ArcaTech]

Closes #134

This makes the changes suggested in the issue. I added a handlePlaceholder method to SearchNode, leaving the possibility for any Node type to do something with its placeholder text. The way I'm calling it in lib/ui/outline.tsx isn't consistent with the way it's called in the rest of the component, but I wasn't sure of the best way to have placeholder be available for the check on L160. That check (to decide whether to show the handle or not) was added because it looked weird when the placeholder was showing, but the handle wasn't.

I feel like there might be a better way of doing this so let me know what you think.

[progrium]

Yea, maybe not ideal, but seems good enough. We can always change it later. Congrats on making your first hook in the system.