Show tab bar if the setting is true or there is more than one tab

[50Wliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

This PR changes updateTabBarVisibility to hide the tab bar if there is less than two tabs, regardless of how many panes are open (as the tab bar is per-pane).

Example:

I inlined shouldAllowDrag as it was only being used by updateTabBarVisibility, and not even for dragging purposes.

This probably won't be merged until I also implement https://github.com/atom/tabs/issues/275#issuecomment-191879765, to easier facilitate drag-and-dropping. See GIF!

Along the way, I also fixed some very incorrect test assertions about the alwaysShowTabBar setting.

Alternate Designs

None.

Benefits

More consistent experience when "Always Show Tab Bar" is disabled.

Possible Drawbacks

This removes the ability to drag tabs out of a pane that only has one tab.

Applicable Issues

Fixes #275 Inspired by #277 Needs atom/tree-view#1293 to work correctly with the Tree View

[daviwil]

Hey @50Wliu, looks like a recent commit you made created a merge conflict that needs to be reconciled with the changes in this PR:

https://github.com/atom/tabs/commit/d6947e37a01573877a5b1be1647cabe6efa74e55

If you can update the PR to take care of that conflict, feel free to merge it afterward. Thanks a bunch!