_mergeSettings(...newSettings) {
        Object.assign(this.settings, ...newSettings);
    }
Merge the `newSettings` given as parameters into the current element settings.

WARNING: Using `Object.assign()` here means the merge is not recursive and only one depth is merged.
cf. http://stackoverflow.com/a/39188108/2834898
cf. tests on http://codepen.io/AnotherLinuxUser/pen/KaJORq?editors=0011

@param {object} newSettings
@private