Increase minimum Qt5/KF5 versions and fix several deprecations
Since Jolla relies on Calligra and has Qt 5.6 only, I limited this update to Qt 5.6 and KF5 5.30, released approximately at the same time.
There is still a dependency on KDElib4Support that is going to be hard to break (KLocale, KCalendarSystem…). As long as there is this dependency, settings the QT_DISABLE_DEPRECATED_BEFORE flag seems to be complicated since this dependency adds a QT_DISABLE_DEPRECATED_BEFORE=0 flag already. We could add a remove_definitions(-DQT_DISABLE_DEPRECATED_BEFORE=0)
, but I am not sure how safe it would be.
We still have pages and pages of deprecation warnings in the whole code base, but we have to start somewhere.