Be sure to not register again asset with same url, create a-assets if missing

[vincentfretin]

Steps to reproduce:

• select yellow cube
• go to the material's src property and click the button to open the textures modal
• right click on crate image and copy url
• click LOAD TEXTURE
• paste the url and press enter (or you can also click on the crate image)
• it currently shows the "crate" name and pressing LOAD THIS TEXTURE will add a second time that asset with a different name. That's wrong.

What it should do is showing the existing asset id "crateImg" and not "crate" to not register the same asset a second time.
I now show the message "Texture already loaded" and the "LOAD THIS TEXTURE" button stays disabled in this case.
I also now handle the case "Name already taken by another asset or entity".

The code

if (image.type !== 'asset') {
  var assetId = insertOrGetImageAsset(image.src);
  value = '#' + assetId;
}

that is executed after selecting a texture that closes the modal probably took care of reusing the same asset in the past but it was a dead branch in the current code, image.type is always "asset" here because props.onClose(value) is called from selectTexture that is used only on registered assets.
I removed that code and the insertOrGetImageAsset function and reintroduced a check in the load texture step to check for existing asset with the same url.
I removed the duplicate insertNewAsset/getFilename functions and moved getFilename, isValidId, getValidId, getUrlFromId and added new getIdFromUrl to assetsUtils.js to be reused with other type of assets and not only img in the future (like selecting an audio for the sound component).
The a-assets element is now created if it doesn't exist, it gave an error previously.
The input field and preview is now cleared if you click on the BACK button.

[vincentfretin]