Implement a HTML-like placeholder mechanism for ScenarioGridModel

Update StringColumnRenderer to support placeholder when cell value is empty.
Placeholder message should be defined at GridCell level and it should have a different style/color (grey + italic)

Acceptance criteria

• All the columns should be created empty and with the placeholder
• getValue of a cell with the placeholder should not contain the placeholder text
• If a cell is edited to remove its value the placeholder should appear again