Allow customization of Job icons

[strangelookingnerd]

In a discussion with @jonesbusy in jenkinsci/custom-folder-icon-plugin#387 I stumbled upon custom-job-icon-plugin which by now has become deprecated / out-dated but brought up the interesting idea of being able to change the default icon of a Job from the BallColor to something else similar as custom-folder-icon-plugin already does it for AbstractFolder.

Since I really liked the idea and already have a use case myself I wanted to bring this to the core to lay the foundation.
Once this is done I'd like to adopt the custom-job-icon-plugin and build up on it.

This PR introduces the icon field of type JobIcon to a Job. It is used in the relevant *.jelly files of the core to visualize a Job and by default relies on a new implementation BuildStatusIcon.
A Job now has a configuration for its icon. Overall the implementation is very similar to the approach of cloudbees-folder-plugin.

Screenshots:

Main View

Job Overview

Configuration

Full disclaimer, I am aware that this draft is still a little rough on the edges how ever to see if this goes anywhere I'd be grateful for some feedback.

Testing done

Only manual testing so far, tests to follow in case we want to follow up on this draft.
For testing purposes I also included the TestIcon class to have something to work with other than the default.

Proposed changelog entries

• Allow customization of Job icons

Proposed upgrade guidelines

N/A

Submitter checklist

Beta Give feedback

1. The Jira issue, if it exists, is well-described.
2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

N/A

Before the changes are marked as ready-for-merge:

Maintainer checklist

Beta Give feedback

1. There are at least two (2) approvals for the pull request and no outstanding requests for change.
2. Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
6. If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[jonesbusy]

Thanks! I really like the idea!