Activity specific unit sprites
Requested (among other things) by elefant in https://forum.freeciv.org/f/viewtopic.php?p=107012#p107012
One thing we can do with current codebase is to make unit sprites specific to their activity.
Depends on #46388
One thing to note is that this can't distinguish activity *targets*, so one can't have different sprites, e.g., for Road and Railroad building as both are "Road" activities.