enable grouping on tablet

I think grouping is genuinely missing from actions in mobile app currently.

I think grouping - and the lo-fi but effective spatial layout it makes possible is

1. an early USP of Capacities
2. the next basic sorting/structuring mechanism of body content, following headings
3. one of the few - but needed - ways to imsert some user side spatial control ("creativity") imto an environment that otherwise often leans to database logics

This is a re-submission of a ticket that was automatically closed after things were initially "under review" (not sure whether the review contributed to dismissal, as there was no rationale stated alongside routine closure)

– see original ticket: https://capacities.canny.io/feedback/p/mobile-app-provide-action-to-combine-blocks-into-a-group