Presenting of UIAlertViewController inside ToolboxCategoryViewController

[vicng]

A UIAlertViewController is being presented by ToolboxCategoryViewController#workspaceViewController, but this causes an Xcode warning:

Presenting view controllers on detached view controllers is discouraged <BKYWorkspaceViewController: 0x7fd914412920>.

Couple of things need to be fixed here:

1. Call addChildViewController(workspaceViewController) on ToolboxCategoryViewController
2. WorkbenchViewController needs to be refactored so it can manage presenting this popover. It currently has special state checks which automatically dismiss popovers unless it's aware of them. (Note: this isn't current behavior, but once Re-added support for CocoaPods/Carthage after changing repos. Updated… #1 is fixed, the workbench will cause automatic dismissing of the alert view.)