[JENKINS-16339] Display Mode configuration options.

Select one of these display modes:

- Truncate (legacy behavior): if the Job name is
  too long, truncate it and show "..."

- Fixed Width: same Widget width for all job name's
  lenghts. If the job name is too long, it will
  wrap to next line.

- Jenkins: act as a regular Jenkins Widget. If the job name
  is too long, it will extend the Widget's width.
[JENKINS-16339] Help file for the Display Mode configuration.

The descriptions could be better...
Set and get displayMode in config page.

Default value is 1 (Truncate)
Implemented different display modes.