remove old street elements before add new elements

[Algorush]

in some cases, when adding a new scene, new street elements are added along with the old ones. Its better to clean up street and building elements before adding new ones.
I added the code to remove the .street-parent element in this PR: https://github.com/3DStreet/3dstreet/pull/355/files#diff-bfe9874d239014961b1ae4e89875a6155667db834a410aaaa2ebe3cf89820556R39
Still need to do this for .buildings-parent element

[kfarr]

@Algorush can you provide more detailed instructions to reproduce is issue of "new street elements are added along with old ones?"

[Algorush]

For example, when changing the street component parameters for default-street element in the editor on the right panel.

I described it a earlier here and suggested a solution for this: #355 (comment)
not the best solution. Because every time you change the parameters of the street component, the entire scene will be reloaded. We can change parameters without reloading the scene, but this requires a little more time to implement

[Algorush]

Created PR to fix this: #448

[Algorush]

I noticed one thing with the checkbox in the right panel interface. The switch doesn't work when I click on an area outside of the small circle. I created an issue in 3DStreet-editror with a description: 3DStreet/3dstreet-editor#374

[kfarr]

#448