Clean up code in AddLayerPanel component and drag and drop

[vincentfretin]

This is mainly code cleanup and creating pure functions (not using variables from outside the function) to better understand this code.
I fixed some issues and implemented drag and drop.
This closes #576

• Don't recreate AddLayerPanel each time we want to add an element, and make sure getSelectedMixinCards is not recalculated unnecessarily.
• Don't create preview entity each time the panel open.
• Don't set y to 0.2 for preview on non segment (for example bicycle on waterfront :p) and properly use worldToLocal when creating the entity so the preview and created entity are at the same position.
• Placing a second element moved the existing custom-group entity instead of moving the second entity inside the existing custom-group entity.
• The preview entity wrongly used a previous position instead of scene origin when no entity was selected.
• Instead of scene origin, place element at the intersection of ground and current camera position.
• Select created entity.
• Allow to drag and drop card to create an element at the drop position

[vincentfretin]

@kfarr Drag an drop implemented, creating the element in 3dstreet-container.
You can add your cursor and it's good to merge.