The old behaviors was that only one sequence is previewed, when
starting to scrub in the VSE while hovering over a sequence.
@Hjalti Hjálmarsson (hjalti) and others found this behaviors annoying because it is
often triggered accidentally.
This patch changes the behavior. Now all sequences in the current
frame will be previewed by default. The old behavior (which can
be useful in some circumstances) is still available by holding ctrl.
One alternative would be to have a setting in the sequencer or
preferences that would switch between these two behaviors. I don't
think this is a good solution, because in some cases the behavior
has to be switched quite often.
Another alternative is to remove the feature entirely. Based on
feedback by hjalti this feature is almost never used, but maybe
some users depend on it a lot. We could also provide an alternative
to the feature. Something like a way to hide all sequences except one.
This patch implements a new parameter to the anim.change_frame
operator. It works okay, but is kinda bad because sequencer specific
details leak into this generic operator (although that was the case
before as well). Imo, a better solution would be to have more specialized
change-frame operators for different editor types, but that is a
bit out of scope for this patch.