Cleanup tab drop event

Will merge this if [atom/tabs PR #89](https://github.com/atom/tabs/pull/89) goes through

Currently, the tab-bar-view stops propagation of its drop event. It's tricky for us to bind to that event, because when our PreviewTabsPaneController is instantiated, the tab-bar may not have rendered, yet. This forces us to [delay binding to the event](https://github.com/ahuth/preview-tabs/blob/926d1551cb33dd043bb842c4c17226ad0ccffdfd/lib/preview-tabs-pane-controller.coffee#L14-16).

Should the PR go through in atom/tabs, we can [bind to the drop event normally](https://github.com/ahuth/preview-tabs/blob/926276d4269b6e9abca208d0e36284ff412213be/lib/preview-tabs-pane-controller.coffee#L13), greatly simplifying the PaneController and making it easier to test.