Fix/improve checkbox

[kulmann]

With raw checkbox inputs it's possible to create a group of checkboxes (v-for) to track which elements are selected.

• create an array for selection tracking
• create a set of checkbox (v-for)
• set the selection tracking array as v-model
• set the item from the iterator as :value
• (add a label rendering something for the checkbox)

The result is, that Vue syncs the selection state from the checkboxes into the array. With our oc-checkbox component, this functionality is lost, as we only accept a boolean value.

Reference: https://vuejs.org/v2/guide/forms.html#Checkbox

[kulmann]

Implemented in #890