private void nhânViênToolStripMenuItem_Click(object sender, EventArgs e)
 {
     NhanVien frm = new NhanVien();
     frm.MdiParent = this;
     frm.Show();
 }
 private void buttonItem3_Click(object sender, EventArgs e)
 {
     NhanVien nv = new NhanVien();
        nv.MdiParent = this;
     nv.Show();
       //  this.Hide();
 }