UsdView: Display resolved labels in prim rollover window

[nvaszabo]

Description of Change(s)

This PR uses the usdSemantics schema and API to compute the taxonomies and the associated labels and display them in UsdView's prim rollover window.

The resolved labels for a prim under the cursor and for its bound materials are computed as follows:

• The taxonomies and their labels on the prim and all its ancestors (inherited labels).
• For each taxonomy, the unique set of labels is computed and sorted lexicographically by the 'UsdSemantics.LabelsQuery.ComputeUniqueInheritedLabels()` API.

Note that the list of taxonomies and resolved labels is truncated to fit in the rollover window, using the new TruncateMiddle() utility function, by omitting the middle part of the list and displaying its start and end. This provides a user experience similar to that provided by the Qt widgets used in the attribute editor.

Link to proposal

N/A

Fixes Issue(s)

N/A

Checklist

[x] I have created this PR based on the dev branch

[x] I have followed the coding conventions

[ ] I have added unit tests that exercise this functionality (Reference:
testing guidelines)

[x] I have verified that all unit tests pass with the proposed changes

[x] I have submitted a signed Contributor License Agreement (Reference:
Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10504

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[jesschimein]

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