[NUI] Add SelectionChanged Event (#3397) (#3449)

[NUI] Add SelectionChanged Event (#3397) (#3449)

Add SelectionChanged event, which will be called every time the selection has changed.

editor.SelectionChanged += (s, e) =>
{
    e.OldSelectionStart; // selection before the change
    e.OldSelectionEnd;  // selection before the change
};

This reverts commit 3cdf1f2609b3dd00a4ca2fee04927e7cd0cba070.