settings: Fix MSVC issues

According to https://stackoverflow.com/questions/469508, we run into a
MSVC bug (since VS 2005) when using diamond inheritance for
RangedSetting.

This explicitly implements those functions in RangedSetting. GetValue is
implemented as just calling the inherited version. The explicit
converson operator is reimplemented. I opted for this over ignoring the
warning with a pragma since this specifies the inherited behavior, and I
have now less faith in MSVC to pick the right one.

In addition, we mark destructors as virtual to silence what I believe is
a fair MSVC compilation error.