show icons for a centre

[gacafe]

• [ ] highlight centres of the accessible-all computation - when doing accessible-all calculations, sometimes we have it as "centres" ie. we are not computing accessible from all buildings (as is default in the ui look) but rather, accessible from particular centres.
• [ ] make a list of those centres as a floating box
• [ ] be able to give the collection of centres a name
• [ ] be able to have multiple collections of centres (at least two)
• [ ] added bonus if one can see those centres even at super high zoom