..._HaikuWidget_delete(): Fix signature
HaikuWindow: Set root view min and preferred sizes

Otherwise MinSize() and PreferredSize() would return indetermined
values.
Haiku: Add paint listener support

* Move some initialization (calling that from HaikuJNIContext::Init()
  and HaikuUtils::Init()) to the new native OS.init() which is called
  right after loading the native library.
* Expose some Haiku constants via the OS class, so they can be passed
  directly to the native methods. Currently only B_ALIGN_*.
* Move HaikuViewComposite class into its own header.
* Control: Call the new HaikuControl.setPaintStyle() which applies the
  SWT styles related to drawing to the target. Currently only
  SWT.NO_REDRAW_RESIZE and SWT.NO_BACKGROUND are supported.
* Add a callback for BView::Draw() and add paint listener support in
  Control.
* Add very basic GC support. Can currently only be initialized with a
  handle provided by the BView::Draw() callback. Only supports setting
  the foreground color and drawing rectangles and strings yet.

Gets the HelloWorld5 example working.