RT Backends: store configuration settings

[pedrolcl]

The back-end plugins may have a function to store the settings, without needing the help of the configuration dialogs provided by drumstick::widgets. This is needed to store a full set of settings in RT projects, like VMPK.

public slot:
    void writeSettings(QSettings *settings);

This method would not be part of the interfaces before Drumstick-3, and could be called only by QMetaObject::invoke().

[pedrolcl]

This ticket is required by /pedrolcl/VMPK/issues/18

[pedrolcl]

Another backend that needs this functionality is macsynth