Gets the SelectedItem of
C1ComboBox.
'Declaration
Public Overridable ReadOnly Property SelectedItem As C1ComboBoxItem
C1ComboBox c = new C1ComboBox();
c.Items.Add(new C1ComboBoxItem("test1"));
c.Items.Add(new C1ComboBoxItem("test2"));
c.SelectedIndex = 1;
// item will refer to test2.
C1ComboBoxItem item = c.SelectedItem;