Easy selection of more multipolygons

[kempelen]

When working with buildings that are multipolygons, it's very difficult to modify more than one in one pass. This task is very simple with closed-way buildings, because I can select more with Shift or Ctrl.

The solution could be:

• Allow multiselect on the list of "Member of" in the "Properties" panel

The usage would be:

• Select parts (e.g. outer line) of the affected multipolygons
• Select their parent relations in "Member of" list (which currently displays all related relations, so it's cool)
• Right click and choose "Select relation"

This would work exactly the same as works with one object currently. But multiselect would make this possible for multiple relations.

Of course this can be used for other things than buildings too.

[akks]

In 5800/josm:

Properties toggle dialog: allow to select and copy/delete multiple keys/values, see #7846
fix #6917, fix #7895 - select multiple "member of" (fixed one non-working action in previous revision)