[NUI] Fix Switch selection bug. (#1798)

[NUI] Fix Switch selection bug. (#1798)

Previously, when a Switch is selected by code, the thumb is not moved to left/right.
```
var switchButton = new Switch()
{
  IsSelected = true,
}
```
Thumb is moved only if the user touches the switch area or presses return key.

This patch fixes this problem and to do that, it needed to change belows,

* Switch does not listen Touch/Key event anymore to detect selection changed
  Instead it uses ControlState changed event which is clearer.
* Button produces more complicate combined state such as "SelectedPressed".
* Clean up the code that is redundant in Selector.GetValue().

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