Changing TextField.Text does not clear text selection. Resulting in `ArgumentOutOfRange` exception in some situations.

Describe the bug

This occurs when you select a range in a TextField then change the Text to a new shorter string value and attempt to update the CursorPosition:

To Reproduce

[InlineData("a")] // Fails
[InlineData ("aaaaaaaaaaa")] // Passes
[Theory]
public void TestSetTextAndMoveCursorToEnd_WhenExistingSelection(string newText)
{
	var tf = new TextField ();
	tf.Text = "fish";
	tf.CursorPosition = tf.Text.Length;

	tf.ProcessKey (new KeyEvent (Key.CursorLeft, new KeyModifiers()));

	tf.ProcessKey (new KeyEvent (Key.CursorLeft | Key.ShiftMask, new KeyModifiers { Shift = true }));
	tf.ProcessKey (new KeyEvent (Key.CursorLeft | Key.ShiftMask, new KeyModifiers { Shift = true }));

	Assert.Equal (1, tf.CursorPosition);
	Assert.Equal (2, tf.SelectedLength);

	tf.Text = newText;
	tf.CursorPosition = tf.Text.Length;
}

Test fails with:

   Source: TextFieldTests.cs line 1303
   Duration: 25 ms

  Message: 
System.ArgumentOutOfRangeException : Index and length must refer to a location within the string. (Parameter 'length')

  Stack Trace: 
String.ThrowSubstringArgumentOutOfRange(Int32 startIndex, Int32 length)
String.Substring(Int32 startIndex, Int32 length)
TextField.PrepareSelection(Int32 x, Int32 direction) line 1139
TextField.set_CursorPosition(Int32 value) line 342
TextFieldTests.TestSetTextAndMoveCursorToEnd_WhenExistingSelection(String newText) line 1318
RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)

Workaround is to call ClearAllSelection (); before updating CursorPosition.

[BDisp]

I think is better change the issue title to something like the or whatever you think better:

TextField.Text setter doesn’t clearing text selection if not empty.

[tznind]

Perfect!