Rework PaneItem management

[smashwilson]

Another bit of ambient cleanup work. This is me trying to get our PaneItem and opener management consistent and React-y. If we like it, I should be able to port over all of our existing PaneItems pretty quickly, as we don't have that many.

Here's a spoiler of what I'm building toward:

render() {
  return (
    <PaneItem
      workspace={this.props.workspace}
      uriPattern='atom-github://something/root/{filePath...}?q={search}'>
      {({itemHolder, params}) => (
        <ItemComponent
          ref={itemHolder.setter}
          filePath={params.filePath}
          search={params.search}
          another='prop'
        />
      )}
    </PaneItem>
  )
}

Fixes #1405.

Pane items to convert

• FilePatchController
• IssueishPaneItem
• GitTimingsView

[smashwilson]

@kuychaco @annthurium: I'm curious if you two like the API I've come up with here, and if you think it's a net benefit from the way that we used to manage pane items.

For reference, here's the new style:

• PaneItem is the major bit;
• URIPattern is a support class;
• IssueishPaneItem is an example of an item component, with this section from RootController as an example of the actual <PaneItem> tree.

The old style had a few different variants:

• FilePatchController used a PaneItem and some state within GithubPackage.
• IssueishPaneItem and GitTimingsView used variants on creating a detached DOM node and rendering independent React trees within their own static methods.

[smashwilson]

😆 Sorry about that, @BinaryMuse. I'm guessing I was overzealously clicking on suggested reviewers?

[annthurium]

yay documentation!!

[annthurium]

looks good to me!

[kuychaco]

👍👍

[kuychaco]

Looks good! Thanks for doing this @smashwilson ✨

[smashwilson]

Cool, thanks for the sanity check 😄