kcms/rules: Change opacity defaults to 100%
This has the benefit of providing a saner default for the values
of opacityActive and opacityInactive rule properties, both
in the UI and when reading the config.
The side effect is the same we have when changing default values: if someone would have set a rule with some opacity to 0%, it will be read as 100% next time kwin starts.
In this case it is a small price, as it is always easier to change it back when the windows are visible that restoring it when they're not, specially if it wasn't voluntary (And moreover in the transition to Plasma6)