Refactoring and cleanup of Cocoa NSWindow and NSView observations

Now only perform observation at all if it was VTK that created the NSWindow/NSView.
This avoids receiving many NSView frame change messages when not needed.

Some renaming, new asserts, fix minor type mismatch.
Things are more symmetrical now.

Change-Id: I337afef3a07fe9e33abdfc6e7e714502262bcc91
Merge topic 'cocoa-observation-cleanup' into master

141198ec Refactoring and cleanup of Cocoa NSWindow and NSView observations