after slide-down animation

Replaced deprecated NSSavePanel and NSOpenPanel APIs with newer ones.

The new API NSSavePanel.beginSheetModalForWindow that is required to
show the Sheet, uses objective-c block for completion
handler. This call can't be made directly from Java, as Java doesn't
support the block syntax (^). Hence, it is called from a wrapper
function inside C code in os_custom.c. The wrapper function calls
beginSheetModalForWindow with the objective C block syntax. The
completion handler block is also implemented in os_custom.c which
callsback to the Java
functionFileDialog/DirectoryDialog._completionHandler(). 

Change-Id: Ib03881a68ef7ecaf2132cf91a4137aef8b6fe3fc