It seems that rna_def_effect() is missing a rna_def_input(), resulting
in:
rna_uiItemR: property not found:
EffectSequence.animation_offset_start
rna_uiItemR: property not found: EffectSequence.animation_offset_end
in the console. I'm not sure whether these settings are used for
"effect" strips or not, but I'll leave that to sequencer guys to
figure out if/when they get a report about this.