Add unit tests for UI logic

[pleku]

There should be some unit tests for the UI logic code. Currently there are no tests whatsoever

[mvysny]

I'm prototyping UI tests at the Kotlin port of Beverage Buddy, you can find them here: https://github.com/mvysny/beverage-buddy-vok/blob/master/src/test/kotlin/com/vaadin/starter/beveragebuddy/ui/CategoriesListTest.kt