Fix category hiding in Angular Material

[lucas-koehler]

Previously, the Angular Material category renderer didn't consider categories' visbilities and always showed themy. This prefilters visible categories before rendering them in the template.

Fix unit test by moving the setting of the new UI schema in the jsonFormsService after it was reassigned in the component. With this, when the event is triggered on the subscription, the new schema is already present. This is necessary for the new changes and also fixes the previously commented out assertion.

[coveralls]

Coverage increased (+0.1%) to 84.451% when pulling 9a02eea96de10a0cd7d3ab712795ea9e09ec7189 on angular-category-hiding into 20e9e0fd72bb388611d7387e1f533d66b7839041 on master.