prevents labels from occluding self reference arcs

[sverweij]

what this fixes

• Places all label text above the self reference arc so it doesn't occlude the arc.
• When wordwraparcs=true, the label wraps to the real available width instead of inter entity
  (this was a problem on the last lifeline - note the absence of a part of the last message label in the sample below)

  Sample

  Input chart

  msc {
    hscale="1.3", wordwraparcs="true";

    Alice, Bob;

    Alice => Bob [label="get Bobs' public key"];
    Bob >> Alice [label="Bobs' public key"];
    Alice => Alice [label="encrypt message with Bob's public key"];
    Alice => Bob [label="encrypted message"];
    Bob => Bob [label="decrypt message with own private key"];
  }

Before:

After: