[NUI] AppBar code refactoring to introduce AppBarStyle (#2713)

[NUI] AppBar code refactoring to introduce AppBarStyle (#2713)

Unused namespace System.Windows.Input is removed.

AppBar constructors with parameters are removed to reduce complexity by
adding constructors with style parameters.

private Initialize() is added to remove duplicate codes in constructors.

DefaultTitleContent and DefaultActionContent are added like
DefaultNavigationContent.

SetTitle() is added to set title text to DefaultTitleContent.
AddActions() and RemoveActions() are added to add and remove action
views to DefaultActionContent.

Co-authored-by: Jaehyun Cho <jae_hyun.cho@samsung.com>
Co-authored-by: Jiyun Yang <ji.yang@samsung.com>