[NUI] remove StateValueCollection from Selector (#1975)

[NUI] remove StateValueCollection from Selector (#1975)

State-Value pair is now added to Selector.StateValueList without
duplicate check.

You need to use Selector.StateValueList when custom state-value pair is added
to Selector. and it is now able to add custom state and pre-defined
state in the same initializer.

Before:
```
Selector<string> textSelector = new Selector<string>()
{
    Normal = "Defalut",
    { ControlState.Pressed, "Pressed!" }, // build error
    { ControlState.Focused, "Focused!" }  // build error
};
```

After:
```
Selector<string> textSelector = new Selector<string>()
{
    Normal = "Default!",
    StateValueList =
    {
        { ControlState.Pressed, "Pressed!" },
        { ControlState.Focused, "Focused!" }
    }
};
```

Also, this patch fixes a bunch of CA2227(Collection properties should be read
only) warnings.

Co-authored-by: dongsug-song <35130733+dongsug-song@users.noreply.github.com>