display blank entries in gray color

[jpcima]

Same as OPL3BankEditor, except one important thing:

At startup it's now needed to set the backup like OPL3 does. (or it will popup the dialog of unsaved changes on "Open" at startup)

At the same place I reciprocated how it syncs the global parameters of banks on OPL3 startup. (was deep tremolo/vibrato, here LFO settings)

    m_bank.lfo_enabled = ui->lfoEnable->isChecked();
    m_bank.lfo_frequency = ui->lfoFrequency->currentIndex();
    m_bankBackup = m_bank;