Allow keyboard navigation keys to be customized.

This change adds select* functions. These functions encapsulate the
common methods of interacting with the calendar via the keyboard, so
that it is possible to specify custom navigation keys:

Calendar {
    id: calendar

    Keys.onLeftPressed: {}
    Keys.onRightPressed: {}
    Keys.onUpPressed: {}
    Keys.onDownPressed: {}

    Keys.onPressed: {
        if (event.key === Qt.Key_W) {
            calendar.selectPreviousWeek();
            event.accepted = true;
        } else if (...) {
            ...
        } ...
    }
}

Change-Id: Ic82660f657eaadacd9a170baf07efbabf3eda332
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>