Split up GraphViz class

[clue]

The current GraphViz class used to be god class that encapsulated all GraphViz-related functionality. This was okay where this class originated (as part of the clue/graph package).

Now that this is a separate package, we should split this up into more discrete parts with a single responsibility each. The existing Image and Dot classes are a first step into this direction (though a badly named and documented one).

This ticket serves as a reminder and as a heads up.

Also refs #2.

[clue]

For the reference: I've started looking into this a few months ago and the results did look promising. Unfortunately I've had to pause this due to changed priorities for now.

[llaville]

Hello @clue
What do you think to raise PHP minimum requirements to 5.4
That will allows to use php trait feature to reorganize code and allows more inheritance and change internal code. See article https://andy-carter.com/blog/overriding-extending-a-php-trait-method
I tell that because I d like to integrate cluster support in a more easy way than forking and sync my copy to solve #42
What do think about it

[clue]

@llaville It's an interesting thought, but I don't think this is related to this ticket? I'm not a big fan of using a trait here, but perhaps you can file a dedicated ticket to discuss this in more detail?