Implement behavioral logic

[thomaspeugeot]

Steps:

• add a Impl field to Rect
• add a package "bahavior"
• add a RectOrchestratorImpl struct with method func AfterUpdateFromFront(stage *gongsvg_models.StageStruct, stagedRect, backRepoRect *gongsvg_models.Rect) that calls stagedRect.Impl.AfterUpdateFromFront(stage *gongsvg_models.StageStruct, stagedRect, backRepoRect *Rect)
• add a RectImplstruct that implements AfterUpdateFromFront(stage *gongsvg_models.StageStruct, stagedRect, backRepoRect *gongsvg_models.Rect)
• add a function Connect(stage *gongsvg_models.StageStruct) to plug the RectImpl to the Rect and that hooks Stage.AfterUpdateFromFront to RectOrchestratorImpl

In the behavior function

• Add ShowHorizontalHandles and allow Handles to appears only if true
• Add CanHaveHorizontalHandles and set ShowHorizontalHandles to true if Selected and CanHaveHorizontalHandles
• Add CanBeMovedHorizontally
• Add CanBeMovedVertically
• Allow a control inversion via a RectUpdated(stagedRect, backRepoRect *gongsvg_models.Rect)