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
This is a little bit a hack, because by the time the separators are
added, it is not clear, if the following section is empty, therefore
we need to parse the content of the following section and eval the
expressions to find out, if this is empty
Remarks:
- catch all exceptions when eval'ing statusline
- make sure, that the seperators are highlighted
even when skipping empty regions (highlight group
names need to be adjusted)
- if a section is defined as empty, it will be removed completly from
the statusline. This means, it won't be called on the next update
and may not refresh properly (e.g. when the whitespace check
triggers, therefore, the whitesapce extension has to call an
explicit redraw whenever it is supposed to be refreshed)
