Improvements to flow diagram

[johnsimons]

Fixes #398

Here is what it looks like now:

And the tooltip is also updates see:

[johnsimons]

@distantcam should the saga diagram line also be thinner ?

And the sequence diagram ?:

[sergioc]

@johnsimons the appearance of the lines should be consistent between the 3 diagrams. The timeout line should be 1px wide and use the color #666666.

This is how it should look like in the Sequence Diagram view (note that label also shows really close to connector line):

[johnsimons]

@distantcam this one is ready to go.
I've decided to split further improvements to other diagrams as a different PR as this one already contains quite a few nice improvements