Ensure Display and Color Map tabs appear.

When models are created without any tessellations attached (i.e., creating a session with an empty model), the display and color map tabs never appear because they require a pqDataRepresentation, which is not created until there's a reason to (i.e., a tessellation exists).

Emit a signal whenever a representation is created and have ModelBuilder respond by initializing the tabs.

To validate, create a new polygon model (no tabs appear yet), then create an edge interactively; the tabs should then appear.

Closes #130 (closed)