Rename isValid() to IsSet() in preferences.cpp, add new isValid*() methods.

This MR removes isValid() in favor of naming it isSet(), making it clearer when a Preference is explicitly set. It also introduces isValid*() methods to check whether the stored string can be parsed as the expected data type.

There’s some code duplication, but this keeps the ability to check if a Preference is set without relying on the ambiguity of default values from get*().

If duplication is a concern, we could consider adding separate extract*() methods that return a std::optional.

Fixes: #295 (closed)