Added tools to create and modify bezier objects

The following things are still missing:
  * Icons for the toolbar
  * Shortcuts for the "create bezier*" tools
  * Translations
  * Export to formats other than JSON
I don't really understand why you made this so complicated. Wasn't the issue simply that your previous approach was flipping each polygon around a different center point? All that should be needed is to calculate the center point once based on `mPolygon.boundingRect().center()` and then flip `mPolygon`, `mLeftControlPoints` and `mRightControlPoints` around this same position.