use numbered dataflow labels on seq diagram

[nineinchnick]

Use numbered dataflow labels on seq diagram. This makes it easier to read when comparing seq and dfd diagrams.

Please note that changes here are very similar to what's proposed in #80, that is:

• moving label and color functions to methods of the Element class
• adding shape and display_name methods which makes it even easier to override classes. As a side effect, _dfd() method has been removed from most classes since it's a duplicate now.

This also has the same fix as #93 but that other PR is way simpler and might be merged earlier.

Preview:

[ghost]

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