Over the last couple years (!) UI buttons have moved to derived classes,
meaning we don't need to use the same "a1" and "a2" variables to store
different information. At this point, that information is set specifically
by internal UI code, or functions like `UI_but_*_set`.
These values are only set to their default 0 values now (or -1 in some
non-meaningful cases). This commit removes the values from buttons
and removes the remaining a1 and a2 arguments from the UI API.
d29ce3d531