private void btnFactura_Click(object sender, EventArgs e)
        {
            FrmFactura MiFactura = new FrmFactura();

            MiFactura.MdiParent = this;
            MiFactura.Show();
        }
        private void facturasToolStripMenuItem_Click(object sender, EventArgs e)
        {
            FrmFactura frm = new FrmFactura();

            frm.MdiParent = this;
            frm.Show();
        }