lets say you have 50 level 1 walls after upgrading to town hall 8. you have enough gold to get them up to level 5, but will take an enormous amount of time clicking and upgrading every single on of them.
this feature could be displayed at an upgrading preview, on the side which says
"upgrade all walls to this level?"
and a confirmation button after that in case its an accident.
if you dont have enough money to upgrade all the walls, then it will say
"select walls to upgrade"
this will also limit the accidents for speed-upgrading walls so people wont accidentally upgrade them to a level you dont want to

