update the readme

- add OE_controller::getSelectionBoundingBox function
- rename "select" function to "picking" in OE_interfaceDisplay (more clear name)
- change selection mode into an editionState in OE_interfaceDisplay
- add scale method to curves and stitchs
- add ScaleSelection state and implement the scale selection actions and UI widget
- clean and comment OE_interfaceDisplay
- add BoundingBox::getCenter method
- add OE_display::screenToDocument method

- display the bounding box of the selection