Minor bug in cwTextBox corrected in SelStart handling

To reproduce the bug, this is the code:
```
Option Explicit

Public WithEvents Form As cWidgetForm
Private WithEvents tmrX As cTimer
Private WithEvents txtX As cwTextBox
Private WithEvents btnX As cwButton

Private Sub btnX_Click()
  txtX.Text = ""
End Sub

Private Sub Class_Initialize()
  Set Form = Cairo.WidgetForms.Create(vbSizable, "Matarat DB Tools", , 400, 200)
  Set tmrX = New_c.Timer(100, True)
End Sub

Private Sub Form_Load()
  Set txtX = Form.Widgets.Add(New cwTextBox, "txtX", 10, 10, 80, 20)
  Set btnX = Form.Widgets.Add(New cwButton, "btnX", 10, 50, 80, 30)
  btnX.Caption = "Erase"
End Sub

Private Sub tmrX_Timer()
  Form.Caption = "SelStart=" & txtX.SelStart
End Sub
```
Just write some chars and click the button, the SelStart remains with a value>0. After that, if you write some more chars in the TextBox and try to backspace, you'll find out that does not work.

I think that this correction also emulates the behavior of the VB's TextBox.

Changed some variables in place of properties.