Allow Group and Template Parts to be resized

[jasmussen]

To reduce the reliance on Spacer blocks to set arbitrary heights, we should consider adding resize handles to groups and template parts:

The spacer serves a good purpose, but should not be the driving factor of layout.

[paaljoachim]

Ooooo this is interesting! Thank you for the issue Joen. I happen to bump into it.
@andrewserong @glendaviesnz @jameskoster (I think this could be interesting for you to know about this issue).

[andrewserong]

Thanks for the ping! I think a minimum height block support for group blocks (in particular) would be really useful. There's been some prior exploratory work in #32499 and #33970 by @aaronrobertshaw and @ramonjd — perhaps we could revisit these approaches after 6.0?

[jordesign]

It's now possible to set a min-height on group blocks - but there's no visual UI on the block itself. Will leave this open for that to be considered.