Id: Toolbar button to add members to a relation

[quincylvania]

With the contextual toolbar, we can have an improved workflow for adding members to relations. (Related: #4915)

1. Select a relation via existing methods
   
   
   
   • "Add Member" button appears in the toolbar
   
   
2. Click the button to enter a new mode
3. Click an existing feature to add it

For multipolygons the button could be labeled something friendlier like "Add Part". Selection could also be limited to ways.

Eventually there could also be an option for drawing new member features directly, but that is beyond the scope of this issue.

[DujaOSM]

Actually, I considered posting a proposal for a reverse work order, which could be used in a more general fashion, but I didn't think of all the ramifications:

_Make use of multi-selection._

iD currently only supports multi-selection, but one cannot do anything useful with the list of objects. Now, the potential complexity is immense (group-change tags and whatnot), but you could start small by allowing the selected objects to be added to a relation (or removed from a common relation). At a minimum, iD should prevent adding the same object to a relation twice (in the same role, at least).

[slhh]

@DujaOSM Adding the same object with the same role multiple times is quite valid, at least in case of public transport version 2 routes.