Add option to have panel collapsed on load

[mhkeller]

Describe the feature in detail (code, mocks, or screenshots encouraged)

Currently, you can set a defaultSize to determine the height on load, but you can't default to having a panel be collapsed on load. I wanted to set the defaultSize for when the panel is in its expanded state but have the panel start off on load. I couldn't find any current way to do this with props.

Here's what I ended up doing to solve it:

function togglePane() {
  if (myPane.isCollapsed()) {
    myPane.expand();

    // We can't set a default size of 50 and also have it be collapsed on load
    // so the first time we expand it, we set it to 50
    const currentSize = myPane.getSize();
    if (currentSize === 0) {
      myPane.resize(50);
    }
    
  } else {
    myPane.collapse();
  }
}

It would be helpful if there were a collapse prop on the component so I could do something like this instead:

<Pane defaultSize={50} minSize={0} collapsed={myPaneIsCollapsed} />

What type of pull request would this be?

New feature

[huntabyte]

Couldn't this be accomplished like this?

<Pane defaultSize={myPaneIsCollapsed ? 0 : 50} minSize={0} />