clutter-frame-clock: Merge states DISPATCHING and PENDING_PRESENTED

Daniel van Vugtrequested to merge
vanvugt/mutter:presenting-state into master

Chronologically they already overlap in time, as presentation may complete in the middle of the dispatch function, or otherwise are contiguous in time. And most switch statements treated them the same already. So they're easy to merge into a single PRESENTING state.

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