Improve rectangle selection

[kintel]

QGraphicsView supports basic rubber band selection, but it's limited:

• Extending an existing selection doesn't work
• It looks like it uses bounding box selection as default. Not sure if this can be extended

When importing many objects, rectangle selection would be convenient. If we do this, QCad's selection style would be cool:

• Drag from left to right: select objects fully inside the drag rectangle
• Frag from right to left: select objects partially inside the drag rectangle