Make hybrid tracings the default

[daniel-wer]

Followup from #3288.
At some later point we should:

• remove the "Create Skeleton/Volume Tracing" links and rename the "Create Hybrid Tracing" link to "Create Tracing" (or make hybrid tracings the default)
• introduce restrictions to allow/disallow skeleton or volume tracing (for tasks) Introduce restrictions to allow/disallow skeleton or volume tracing in tasks #4747

[philippotto]

• ensure that the user can decide between "with" and "without" fallback layer Creating hybrid tracing should ask for new vs. existing segmentation #4051

[normanrz]

• enable isosurface rendering in hybrid tracings (also see Isosurface for volume annotations #3681)
• undo for volume annotations (see Add undo for volume annotations #4001)
• drag'n'drop import for hybrid annotations (see Drag'n'drop import for hybrid annotations #4347)

[fm3]

• Another issue here is that of mergeability. Skeleton-only annotations in collections (such as projects) can be merged into a compound view, while this is not defined for volumes. Compare discussion here: https://discuss.webknossos.org/t/handling-of-volume-tracing-projects-with-many-tasks/1500/4 (also see allow merging of volume tracings #3870)

[fm3]

• remove the “use existing segmentation” dropdown menu (default to yes, add the option to remove it later) (see Allow to remove fallback layer from a volume tracing #4741)

[philippotto]

• take care that "create annotation" still works for datasets with uint64 segmentations (right now, wk defaults to creating a fallback segmentation which fails). also see: Fully support uint64 segmentations and volume annotations #4852

[fm3]

If we still want to give the user full control there are quite a few options. Will probably look something like this, what do you think?

or

I’m not sure how to call the options… some of them will probably be disabled for some datasets (e.g. if there is no segmentation layer or if it is 64 bit)

[philippotto]

@fm3 Thank you for the mock-ups & the inspiration! I went with a cascading menu and only put the "create annotation" items in the menu. Let me know in case you have objections :) PR: #4939