Impossible to get IDs of labels assigned to a `GHIssue`

[dmitrykuzmin]

When doing issue search, the results are provided as GHIssue which, via getLabels(), can provide own label list.

The problem with this is, the getLabels() method returns List<GHLabel>, and GHLabel doesn't store the unique numerical ID assigned to the label by GitHub. This makes API usage problematic for apps that want to utilize the label ID and cannot rely on its mutable parts like name or URL.

[gsmet]

I created #1062 to fix this issue.