Qvariant improvements

[direc85]

• Add to_qstring() wrapper
  • More versatile and efficient than .to_qbytearray().to_string()
  • According to documentation e.g. QVariant(QColor) shouldn't be toString()able but it is
• Add From<QVariantMap> wrapper
  • Can save a move/clone on Rust side
• Improve debug print
  • Add type_name wrapper
  • Include the type QVariant contains with printable value, if any
  • Makes debugging QML<>Rust interactions easier

In Whisperfish for example debug prints like this are now possible:

tracing::debug!(tracing::debug!("Attachment: {:?}", attachment);
// Attachment: {data: QVariant(QString: "/home/defaultuser/Downloads/test.md"), type: QVariant(QString: "text/markdown")}

Whisperfish!570 currently depends on this (but it can (barely) do without)