[NUI] Add default components styles and etc (#1396)

[NUI] Add default components styles and etc (#1396)

* Add default styles for : Progress, Slider, Toast, Popup, DropDown
* Fix : PopupStyle.Buttons.Size.Height works now
* Add : Popup Dismiss() method
* Fix : Popup should be posted on the top layer
* Fix : DropDown button did not form well before first click
* Fix : DropDown selectedItemIndex did not update when select an item by touching it
* Fix : DropDownItemStyle did not applied to DropDownItemView so it was not able to detect control state change
* Fix : Progress need to show buffering bar and progress bar together in Determinate stage
* Fix : View.BackgroundImage property would reset its content when it gets null or empty string

Signed-off-by: Jiyun Yang <ji.yang@samsung.com>