For some (unknown) reason [NSApp postEvent: ... atStart: NO]
doesn't append the event, if the message queue is empty
(AKA [NSApp nextEventMatchingMask .. ] returns nil).
Due to nextEventMatchingMask usage, these postEvents have to
run in the main thread.

Using performSelectorOnMainThread deadlocks, since the calling
thread may have locked the Yield mutex, so we simply defer the
call using an NSEvent, like the Windows backend.

So we have to peek at the queue and if it's empty simply prepend
the event using [.. atStart: YES].

In the end this make the vcl_timer unit test pass on OSX.

Change-Id: Ib41186425b2f76faa0e9f116f47fdcd60d878099