Progress Controller skips lifecycle events for a ProgressHandle

When progress is started, an event of type PROGRESS_START is generated. But it may get merged with e.g. SILENT or SWITCH events, as the attached log shows.

The net result is that the ProgressUIWorker does not see the proper lifecycle (start .... processing ... finish) but just a part of the cycle.

The fix is beyond my immediate capacity: while two 'progress' events can be eventually merged, events that bring determinate / indeterminate switch or silent / working state changes should not discard the other 'major' states.

Skipping the important lifecycle points makes the implementation of UIWorker much harder.