Fix GhostTab and GhostTabPane array dependencies

No issue
- TabPanes weren't finding their tab due to not listing all their dependencies in Ember.computed

I'm sorry :(

## Issue Summary
Tab panes aren't finding their tabs correctly! Without its tab, a tab pane can't know if it's active or not.

## Repro Steps

1. Go to the PSM
2. Click on the `Meta Data` tab to go to the metadata pane
3. Open the ember inspector, view all components, find the gh-tab-pane, look at its "tab" property. It is undefined

Why?
The tab-pane looks before its tab before either it or its tab have been registered with the manager. That's not a problem, so long as it looks again after the tab manager's tabs and tab-panes lists have been modified, but it's not doing that.

## Fix
First off, if you look at the code for a tab-pane to find its tab, you'll see it is doing a `Ember.get` on two different properties, but only depends on one of them. That's not good. Secondly, `@each` looks at the properties of objects in an array, rather than the length of the array itself, which is what `[]` is for.