JOSM #21944 - Add 'replace selected' action to relation editor

[Woazboat]

Add a new action + button to the relation editor to replace chosen member primitives with objects from selection (while keeping the roles).

[grischard]

Don't forget to open a ticket on the JOSM side and to link the ticket here and this PR there.

[Woazboat]

Ah sorry, forgot to add the reverse link here https://josm.openstreetmap.de/ticket/21944

[Woazboat]

One comment with the replaceselectedright.svg: As we are replacing something, should some portion of the blue bar be red (maybe two pixels or something?).

I'm pretty sure a better icon than the one I quickly added here could be created. Unfortunately I didn't really have any good ideas how to represent replacement (and have limited art skills), so any suggestions or icon contributions are welcome.

[Woazboat]

I changed the icon to be partially red and fixed some checkstyle issues