[skara] RFR: issuetracker: add Label class

[Erik Helin]

Hi all,

please review this patch that introduces the `Label` class in the `issuetracker` package. The `Label` class provides access to an eventual description of the label, previously we only returned the label names. The majority of the patch is a simple refactoring of changing existing calls to `Issue.labels` to `Issue.labelNames`.

Thanks,
Erik

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Commit messages:
 - issuetracker: add Label class
 - wip

Changes: https://git.openjdk.java.net/skara/pull/1089/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1089&range=00
  Stats: 383 lines in 48 files changed: 106 ins; 0 del; 277 mod
  Patch: https://git.openjdk.java.net/skara/pull/1089.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1089/head:pull/1089

PR: https://git.openjdk.java.net/skara/pull/1089