[NUI] Copy, Paste & Cut Public APIs (#3450)

[NUI] Copy, Paste & Cut Public APIs (#3450)

[NUI] Adding the Copy, Cut and paste public APIs to NUI.

Added:
string CopyText() //Copy and return the selected text.
- This function will copy the previously selected string into the clipboard and will return it.
string CutText() //Cut and return the selected text.
- This function will cut the previously selected string into the clipboard and will return it.
void PasteText() //Paste the most recently copied/cut text.
- This function will paste the most recent string in the clipboard stack into the text control.