macosx: Add an internal function to VLCLibraryWindow to set current segment view

This PR provides a method of setting the view that should be currently set on the library window. Unlike segmentedControlAction it does this without appending this to the navigation stack history, and without checking (and returning early) if the cached copy of the selected segment is the same as the segment currently selected in the segmented title control.

• Eliminates compile-time warnings about passing nil to selector that requires not-nullable argument for segmentedControlAction
• Prevents the need for hacky workarounds to not append a state to the navigation history stack after a call to segmentedControlAction
• Fixes an issue with the correct library view not being re-set after the video view -- which is pushed onto the library window, hiding all other views on starting video playback -- is done playing video