This is for https://github.com/LLK/scratch-paint/pull/194, making the selector area smaller on tablet so as to create more space for the paint editor

Prevent switching tabs while one of these blocks is glowing from
crashing because it is given a new ID.

eslint-plugin-import doesn't like these inline loaders.

I tried unsuccessfully to configure the recommended solution with `eslint-import-resolver-webpack`, but we should come back to this.

With this change we only use `xhr` in the `CostumeCanvas` component.

This way the instance can be reused, which allows access to non-deprecated `DataFormat` and `AssetType`.