cubicweb #2811282 update i18n commands to handle selectable uicfgs [done]

i18n commands use uicfg instances to decide which "schema"-related messages should be included in the po file. Now that uicfg are selectable, i18n commands need to iterate on all possible uicfg objects in the registry.

For instance, if there are 4 different appears_in_addmenu instances, we have to consider all possible add <cwetype> actions they define.

prioritynormal
typebug
done in3.16.6
load1.000
load left0.000
closed by#e713c47a993d [devctl] properly generate i18n messsages for cubes that uses uicfg instances (closes #2811282)
patch[devctl] properly generate i18n messsages for cubes that uses uicfg instances (closes #2811282) [folded][devctl] properly generate i18n messsages for cubes that uses uicfg instances (closes #2811282) [applied][devctl] fix crash on i18ncube command (closes #2808251) [rejected]