Form validation

As a Studio app builder, I am warned about any missing mandatory information in my layout

AC

• When [saving my layout / loading my layout] and it contains errors, a validation error is shown
• All widgets in error are clearly indicated as such in the view editor
• When selecting a widget, missing required properties are clearly indicated in the properties tab

• Analytics event is sent when layout is saved / loaded and contains errors
  • Category: Described according to NXS-5742
  • Event: Validation error
  • Valut: Number of errors detected
    (usage: see how often people end up with validation errors so we can reduce that)