Spend time refactoring variable names, types etc

[theophanemayaud]

• Use constants instead of magic strings like in mainwindow.cpp _ui->findDuplicates->text() == "Stop"
• Make functions constants if they do not use state
• Check imports if really necessary and also which ones might be mission
• Investigate not used warnings
• Refactor functions and interactions to make overall program more simple
• Investigate static Db implementation where the Db location is store in static variable and no object created every time for Db access
• in mainwindow.cpp investigate the _everyVideo video pathnames list uses to see if it could be made more efficient, for example while adding videos to list checking for duplicates for(const auto &alreadyAddedFile : _everyVideo) is a for loop each time, not very efficient !
• Find TODO-REFACTOR comments in code for places that might be good to look at