Fix: in Firefox markers not re-rendered when editing after a link click

[sverweij]

• fixes the defect described below by using marker-start/ marker-end attributes instead of style="marker-start: ..."
• unifies the ways paths are written (path strings tend to be a bit shorter now too)
• documented attempts to
  • simplify markers by starting at coordinate 0,0 (looks like it won't render in canvg then)
  • handle varying the line width

    steps taken

    1. in firefox create a sequence chart with at least one arc in it e.g. a -> b
    2. click the 'link' button - the URL now gets updated
    3. add some stuff to the sequence chart (e.g. a second arc)

       expected

• the chart with both arcs on it is rendered

  found

• the chart is rendered, but the marker head of the first arc is gone

  notes

• Only occurs in firefox.
• Subsequent link button clicks + some typing seem to 'forget' where the markers are. Looks like firefox is using the complete URL including parameters to locate the #id of the marker.
• Surprisingly, in the server log you'd see a GET request on the 'some typing' (step 3)
• Annoying in debug-mode which automatically changes the URL and hence removes all markers of previously entered arcs...

[codecov-io]

@@            master    #196   diff @@
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  Files           31      31       
  Stmts         7043    7043       
  Branches         0       0       
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  Hit           6475    6475       
  Partial          0       0       
  Missed         568     568       

Review entire Coverage Diff as of 1cc7fc4

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