previously, it could happen that the same highlighting group was defined
several times, because it was available in several modes within
g:airline#theme[mode]. So the second one would always win.
Therefore, loop through all modes in reverse order and define the group
and remember what group has already been defined. If we happen to have
to re-define the same group, skip it. Since we are traversing the list
in reverse order, this should make sure the last definition wins.
This has the benefit of being more performant and hopefully helps with
e.g. #1779 and similar issues.
b7367c605a