Minor style/documentation update in PLEditorViewController.

Documented the autocompleteViewController instance variable and fixed two braces positions for style consistency.
Tab subviews are now made first responder on tab switching.

In PLTabViewController, make the window's first responder the view controller of the new active tab. Both view extensions controllers (PLEditorViewController and PLInterpreterViewController) implement the NSResponder method becomeFirstResponder. In each, they make their text view the first responder of the window.

Reorganized the order of events in LiasisAppDelegate applicationDidFinishLaunching so that the final step is to set the active tab in the tab view controller, which now assigns first responder.