Setting the start screen

[nathanielw]

Narrative:
As a user
I want to be able to set the start screen for the prototype
So that I can ensure it always starts in the same place when testing and the models I generate are accurate

Acceptance Criteria:

Scenario 1: Required buttons exist
Given a screen is active/being edited in the PIM builder
Then there is a button for the current screen for marking it as the start screen.
moved to #5

Scenario 2: Setting the start screen
Given a screen is active/being edited in the PIM builder
When the button for marking the current screen as the start screen is clicked
Then the current screen is marked as the start screen
And none of the other screens are the start screen