qt history: fix exporting history
qt history: fix exporting history

closes #4948

spesmilo/electrum,vialectrum/vialectrum,neocogent/electrum,pooler/electrum-ltc,wakiyamap/electrum-mona,spesmilo/electrum,fujicoin/electrum-fjc,vialectrum/vialectrum,wakiyamap/electrum-mona,vialectrum/vialectrum,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,neocogent/electrum,fujicoin/electrum-fjc,wakiyamap/electrum-mona,neocogent/electrum,spesmilo/electrum,wakiyamap/electrum-mona,fujicoin/electrum-fjc