[Implement] edit elementDefinition and ElementUsage from Model Editor; fixes #770 and #777

[samatstariongroup]

Prerequisites

• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

• Adds an Edit for the currently-selected element in the Model Editor's right panel. The ElementDefinition form edits ShortName, Name, Owner, IsTopElement, Categories, and the multi-language Definition collection (via the existing DefinitionsTable). The ElementUsage form edits ShortName, Name, Owner. Both commit through ISessionService.CreateOrUpdateThingsWithNotification. Promoting IsTopElement updates the iteration's TopElement atomically; demoting the current top is silently ignored to mirror the delete-blocks-top rule.