macOS: duplicate Qt "Preferences" menu item
macOS: duplicate Qt "Preferences" menu item

There is a standardised location along with reserved hotkey for "Preferences"
in applications on macOS. Let's put *another* preferences menu item there.

The duplicate items ensure that
- an electrum user coming from a different OS,
- a macOS user used to the standardised preferences location,
will both find "Preferences" easily.

pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,vialectrum/vialectrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,wakiyamap/electrum-mona,vialectrum/vialectrum,pooler/electrum-ltc,pooler/electrum-ltc,vialectrum/vialectrum