fix race in preview mode, and fix preview shortcut keys in qt5
fix race in preview mode, and fix preview shortcut keys in qt5

Stvad/anki,hssm/anki,holycrepe/anki,LucasCabello/anki,abeyer/anki,florianjacob/anki,hssm/anki,ospalh/libanki3,Arthaey/anki,jakesyl/ruby-card,weihautin/anki,subfusc/anki,socialpercon/anki-1,eduOS/anki,weihautin/anki,sunclx/anki,jakesyl/ruby-card,kenjhim/anki,xuewenfei/anki,go38/anki,jkitching/anki