Be clearer about showStartAttributes.

[HansOlsson]

Closes #3406

[d-hedberg]

Give tools more freedom to show what makes most sense when showStartAttribute is not specified.

[HansOlsson]

To me this:

• Gives tools the freedom to show start-values when appropriate
• Gives model developers the possibility to override that

The only part that seem controversial is the interaction between values of connectorSizing and showStartAttributes and showing the variable in the parameter-dialog, but I don't see any practical issues with it, only that it looks a bit impure.

As an example consider:

   Real x1 annotation(Dialog(showStartAttribute=false));
   Real x2 annotation(Dialog(showStartAttribute=false,group="Start"));
   Real x3 annotation(Dialog(showStartAttribute=true));
   Real x4 annotation(Dialog(showStartAttribute=true,group="Start"));

giving something like: