Allow Showing Mode Transition Triggers as Labels

[philip-alldredge]

In support of #158 and general layout improvements, we will add the ability to show mode transition triggers as text labels. The AADL standard has examples where triggers are represented both ways. They are currently displayed as dashed connections between the port and the mode transition. This representation has several problems:

• Additional lines can clutter the diagram.
• Connections between connections are problematic. See Fix Connection Anchors #238.
• General purpose connection layout algorithms typically have no concept of such connections.

When implementing #211, we will look into changing the default representation while maintaining backwards compatibility.