The `transformEvent` function is becoming increasingly complex due to
the inclusion of operation-specific code.
To improve this, remove the `handled` boolean and allow each
`TransModeInfo::handle_event_fn` to determine how to handle already
processed events.
Additionally, move some operation-specific logic to the operators file.
Note:
The `handled` boolean was added in aef307cf31 to fix a bug with the
custom events of the Edge and Vertex Slide operators;
6d5d3ce975