Remove unneccessary #import statements
Add safeObjectAtIndex to NSArray

This is defined in OakTabBarView as a static method, but it could be useful elsewhere.
Use new safeObjectAtIndex method

Also removed the static SafeObjectAtIndex mehtod in OakTabBarView
Ensure FSItem name property is not nil when setting menu title

We use to pass nil to NSMenuItem initWithTitle: when clicking the SCMFolders in
SCM Status (see commit e5841fe); however, we will *probably* never encounter
this now. But it might be useful, if custom data sources are implemented for the
file browser in the future.