Translatable strings refactor

Goal

Having most of the UI translatable when a translation system is released.

Dev

Most strings which are only displayed in the UI and whose value are not used for comparison were framed into the QObject's tr() method or QCoreApplication's translate() method.

Notes

• tr() are not preceded by <Base_class>:: because there are many occurences, and they have to use the closest class in order to make translation files sort them in the correct class categories.
• In any case, there should not be any QObject::tr() in the code nor any QAnyClass::tr()
• QCoreApplication::translate() is used when making string outside of QObject-inherited classes or classes that can't have the Q_OBJECT macro