Add popover appearance property

This property allows to switch the appearance of the popover view between NSPopoverAppearanceMinimal and NSPopoverAppearanceHUD.
Extend demo application for the popover appearance feature

Demonstrats the usage of the popover appearance feature by adding a checkbox for switching between the supported appearances NSPopoverAppearanceMinimal and NSPopoverAppearanceHUD.
Add property popoverAppearance to public interface

* remove explicit setter setPopoverAppearance
Refactor popoverAppearance code

* remove popoverAppearance property
* add popover property to public interface (readonly)
Merge pull request #23 from copa/feature/PopupAppearance

Feature/popup appearance