[ign-gui2] Workflow updates

[chapulina]

• Add labeler workflow
• Add triage workflow
• Add Ubuntu CI workflow with Codecov upload
• Update badges
• Port CODEOWNERS

[maryaB-osr]

👍

[chapulina]

Tests are running, but only a fraction of them pass due to #58. Coverage reports are being generated, but like a few other libraries, Codecov is mysteriously not commenting back.

We shouldn't block this PR because of these.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (ign-gui2@a9284b5). Click here to learn what that means.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             ign-gui2      #56   +/-   ##
===========================================
  Coverage            ?   14.56%           
===========================================
  Files               ?       13           
  Lines               ?     1332           
  Branches            ?        0           
===========================================
  Hits                ?      194           
  Misses              ?     1138           
  Partials            ?        0           

---

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[chapulina]

Codecov is mysteriously not commenting back.

Oh hey! Codecov is mysteriously commenting back!