- sometimes a slot is easier than a lambda. Introduce
a macro CALAMARES_RETRANSLATE_SLOT that calls a given
slot in an object on language change.
- extend the retranslator with support for calling slots:
- use Qt's signal/slot mechanism alongside the private
list of functions to call
- provide convenience for obtaining the retranslator of
an object.
In order to avoid deep copies, Qt containers over which we iterate must be const
The remaining unported cases require qAsConst (Qt 5.7) or std::as_const (C++17)
