Put handles on a different layer

[tim-kilian]

Problem

Currently the handles are displayed as shapes. We could put them on a ui layer to ensure they stay the same as the viewport zoom. This could fix #18

[tim-kilian]

Remember to remove the quickfix in the ElementRegistry.

renew-lib/src/core/ElementRegistry.js

Lines 10 to 16 in 21bb278

	/**
	* TODO: Remove hotfix for handles
	* @return {*}[]
	*/
	getAll () {
	return super.getAll().filter((element) => element.type !== 'handle');
	}