Stunner - [Case Modeler] Change the text on empty canvas

The Case Management must contain at least one Stage and all other supported nodes (User Task, Subcase and Subprocess) must be placed only in to already existing Stages.

Currently there is shown this text, when canvas is in empty state:
"Click, drag or interact with a node"
"To create a case management, click or drag a node in the left-hand palette onto the canvas"

There is a need to change the text to reflect that the Stage must be created first.

Acceptance criteria

The shown text for Case Management is:
TODO