Fix ControlConfig::getScale converting scale to int silently
In Wayland, when having a not int value for scale saved in conf for instance 1.5, when opening kcmshell5 kscreen, the scale displayed is truncated to its int value.
Because of the default value in ControlConfig::getScale -1 being an int and the get() template using this type to determine return value type of the function causing a silent conversion from qreal to int.
candidate for backporting to stable branch