Deprecation warnings when compiling with Qt 5.15

[ahndee]

Qt 5.15 deprecated the comparison operators (<,>,<=,>=) for QVariant (see official documentation) which are used in rangefilter.cpp and rolesorter.cpp, resulting in compile-time warnings.

[timangus]

Perhaps QSortFilterProxyModel::lessThan or the implementation of QAbstractItemModelPrivate::isVariantLessThan is useful here?

[thecompez]

It is better to solve this problem before it causes more serious problems!