Blender Git Loki

Overlays: Make flash on mode transfer an operator property

This moves the flash on mode transfer effect option from the overlays to
an operator property of the mode transfer operator.

- This effect is intended to show the target object when no overlays or
a minimal set of overlays is enabled. Making it part of the whole set of
overlays invalidates this use case.

- The effect is not intended to be configurable per viewport, it should
be a global option.

The effect is still implemented using the overlay engine (instead of a
draw modal callback) due to performance and drawing artifacts. Having it
implemented as an overlay with runtime timer data in the objects makes
also possible to run multiple animations at the same time without any
visual glitches.

Reviewed By: campbellbarton, JulienKaspar

Differential Revision: https://developer.blender.org/D11519