Make QPlaceAttribute Extendable

The QPlaceAttribute class only has fields for label and text, where the text is simply a single string representation of the attribute. Ideally attributes should be of arbitrary complexity and should be extendable by third parties.

This would be achievable by adding a key-value map field to QPlaceAttribute where third parties can add their own custom data. Similar to QVariant there could be an enum for “standard types”, something like QVariant::Type and QVariant::type(), and there could also be a function for “custom types”, something like QVariant::userType().