Replace wxNSTimerData -setTimer method with -initWithWxTimer: initializer

Replace wxNSTimerData -setTimer method with -initWithWxTimer: initializer
which correctly calls the superclass (NSObject) init.

Add an -init method for correctness; technically not needed as it is never
called but it's an Objective-C best practice.

Release the wxNSTimerData instance just after passing it to the NSTimer
factory method.  NSTimer retains it and we don't keep a pointer to it
so we should not keep a refence to it.  This fixes the bug in wxTimer::Stop
where the program crashes in the NSCFTimer userInfo method because the
NSTimer has already been invalidated.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35941 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775