qt: use Pearson's hash instead of generating charcode map file for VLC icons

I see no reason to not do this. Pearson's hash is very simple and it can be used instead of generating a map file. Currently the map file generation is not even integrated into the build system (both automake and meson).

RFC-3074 (https://datatracker.ietf.org/doc/html/rfc3074) provides the hash implementation directly, so it can be assumed to be widely available. This is the reason I did not use Qt's qHash().