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.