Replace geometry of node with multipolygon

[joshdoe]

See #7277 for the previous case of replacing a node with a way; this is just doing the same but for a multipolygon. The code will largely be the same, but there may be multiple ways which should be checked to transfer the node. Not sure if I should require the relation to be complete before doing this command, or checking for it to be a valid multipolygon. Also I should only consider ways that have role inner or outer.

The functionality of #7277 has come in very useful while "upgrading" GNIS nodes to ways, but something like 20% of the time I'm dealing with multipolygons, thus the reason for this ticket.

[joshdoe]

Functionality added in ​[o27645], but not yet published. With this plus the faster relation selecting of #7314 and #7317, and the conflict dialog of #7278 has allowed me to merge GNIS nodes with multipolygons very quickly (for parks in my county).