Haiku: Display: Add deferred event support
Haiku: ToolItem: Add selection event support

For drop down items we don't discriminate between button and arrow
click yet.
HaikuButtonToolItem: Fix ToolItemSetSelected()

Would ignore the argument and always select.
Haiku: ToolItem: Add radio group support
Haiku: Widget: Implement getStyle()
Haiku: ToolItem: Discriminate drow down arrow selection event

* HaikuPushButton ctor/HaikuButtonFactory:CreatePushButton(): Add
  bool withPopUp argument specify pop-up behavior. Set a pop-up message
  with a distinct what code.
* Pass the message what code to the button invoked callback.
* Detect the drop-down arrow case in the ToolItem callback.