Modernize metatype handling in Qt WebChannel

The entire metatype conversion of QVariants to internal JS types in WebChannel could use modernization.

Some QMetaType usage is old-fashioned and could need an overhaul such as handling and detection of flags, enums and number types.

Check interaction with the new Qt6 property system, ensure dynamic updates work both ways, and write tests for it.

Fix all warnings, and usage of deprecated methods.