Tween node into rectangular shape hosting its source code

[matanox]

Using this approach (see also here) for the transition, ultimately turn a node into a shape more optimal for showing the source code inside of it (i.e. a rectangular one), and show the source code inside. We already manage an index of source code per node (some elements don't have source code, if they are important from a library rather than included; for those we'll figure the best way to give the user feedback that this is the case).

There's no escape from allowing scroll inside, this can be accomplished I assume by embedding an html piece inside SVG, or by directly positioning an html piece and synchronizing its location into the animation....

When it's ready, we'll consider via experimentation - whether this is an overlay, or it's just a large rectangle that pushes the other nodes aside (thus preserving the context). After the latter we'll also accordingly figure how the shape can be reverted back to its original state.

We'll take care of syntax highlighting later - it won't be your grandmother's syntax highlighting but something based on TF-IDF or on highlighting calls and type relations.