Allow making multiple selected items members of a relation all at once

[hhm0]

It would be totally awesome to be able to select multiple items, and edit their relation properties (for example add all to a relation, delete all from relation, etc.)? This would make it a lot easier to make items parts of relations!

This is a subset to #1761; however, the GUI issue is a lot more straightforward here, since the relations are not specific to each object like tags.

[thibaultmol]

With V3 having added the ability to add tags to all selected items. Having this also work for relations would make sense and be more intuititve I think #1761

[quincylvania]

@thibaultmol I'd definitely like to have more controls dispalyed during multiselection, such as issues and fields. Not sure if this control would be a fit but we can think about it more.

[1ec5]

Recently, I needed a way to select all the members of a relation in order to add them to a new boundary relation that has the same geometry as the existing boundary relation. This is a somewhat common task in OpenHistoricalMap, where the boundary of a single jurisdiction may need to be modeled as multiple relations, each representing a different time period and mostly similar geometry.

This workaround is clunky, but a lot less time-consuming than the alternative (and less error-prone than resorting to Level0):

1. Zoom to fit the whole relation or z15, whichever is more zoomed out. The relation’s member ways remain visible, but don’t select any of them, or else the others will vanish.
2. Hold down Shift while dragging to draw a lasso around everything in view. This selects the vertices of each member way.
3. Deselect each standalone point that doesn’t have a parent way by shift-clicking on it, or by clicking the ✖️ button in the list at the top of the left sidebar.
4. Press Ctrl+↑ (⌘↑ on macOS) to select the parent ways. This only works if all the selected features have parent ways, hence step 3.
5. This may also select some ways that are connected to the relation’s members but don’t themselves belong to the relation. Again, deselect these ways by shift-clicking on them, or by clicking the ✖️ button next to each of these features in the sidebar. You can hover over a feature in the list to highlight it on the map and vice versa.
6. At the bottom of the left sidebar, add the selection to a new relation.
7. If all the members need the same role, such as outer on a boundary relation, repeat steps 1–5 and enter the role into the new relation’s row in the sidebar. Alternatively, create the relation beforehand and enter its ID into the relation selection combo box (such as r-2 for the second relation you’ve created during this session).