Add an AsyncQueryModel fit for use with QML's TableView

[misje]

TableView uses roles to access column data, so the model needs to provide roleNames() and an implementation of data(…) that uses the role integer for column access. Either let the user specify the role names (and automatically assign them role integers starting from Qt::UserRole + 1) or somehow fetch the role names automatically from the query (if possible).

Since the implementation and complexity will differ from the original AsyncQueryModel, create a new class.

[misje]

Class added in b040c9b. Documentation and example code missing.