consider a window with these splits:
,----
| file1
| ---
| file2
| ---
| file1
`----
If the top buffer is the active one and you start modifying this buffer,
this will also reset the highlighting for the inactive buffer2, since
the highlighting group 'airline_c_inactive' is used for both windows
(one having the unmodified buffer 'file2' and one having the modified
'file1').
This lead to the incorrect highlighting of the buffer name of file2.
Airline basically already created different airline_c<bufnr>_inactive
highlighting groups, but unfortunately did not use them.
Therefore, make the builder aware of this and always append the buffer
number to the group 'airline_c' if it is in an inactive window.
2) we need to make sure, the highlighting won't get overwritten, so
make the highlighter aware of this situation as well, by appending the
buffer number to the group name, if it creates the 'inactive' mode
groups and a buffer number has been given.
this fixes#1233
73aea86a7a