Show blocked requests properly

[DBX12]

Description

This PR adds a new connection type Alice -x Bob which represents a request which was cancelled before it reaches Bob.

Fixes #13

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• Running existing tests
• Created new tests

Checklist:

• My code has been linted (make lint)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have rebased my branch to include the latest changes from master

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I think adding an explanation what a A -x B connection represents should be documented, but I'm unsure about the form. Modifying the code which draws the legend might lead to merge conflicts with the changes on PR #14

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