[NUI] Add GrabHandleColor property (#2776)

[NUI] Add GrabHandleColor property (#2776)

GrabHandleColor property can set the color of grab handle in TextField/TextEditor.

exmaple)
field.GrabHandleColor = new Color(0.0f, 1.0f, 0.0f, 1.0f);

this patch should be merged after below patches are applied.
https://review.tizen.org/gerrit/#/c/platform/core/uifw/dali-toolkit/+/255726/
https://review.tizen.org/gerrit/#/c/platform/core/uifw/dali-csharp-binder/+/255739/

Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>
[NUI] Use Color type instead of Vector4 for GrabHandleColor

Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>
[NUI] Add missing code for GrabHandleColor

field.GrabHandleColor = new Color(0.0f, 1.0f, 0.0f, 1.0f);

Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>
[NUI] Replace Vector4 with Color in TextField/TextEditorBindableProperty

Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>