vim: Make trailing space highlighting more subtle

I sometimes work in files where I cannot remove trailing whitespaces (at
least not permanently). In these cases the background highlighting of
them is quite strong and can be a bit annoying. To make this more
pleasing, use the foreground to highlight trailing characters, as long
as tabs and trailing spaces are displayed as non-space characters.

For this I also set `trail` in `&listchars`, and while at it, merged the
two lines setting `&listchars` and cleaned up some comments.

.config/vim/vimrc.d/30-settings.vim

@@ -77,11 +77,11 @@ if (has('cmdline_info'))
 endif
 " Show whitespace characters
 set list
-set listchars=tab:>·
+" Display tabs and trailing space characters as well an indicator for long lines
+" when not wrapping
+set listchars=tab:>·,trail:·,extends:>
 " Wrap lines
 set wrap
-" When wrap is off, show a visual indicator that the lines continues
-set listchars+=extends:>
 " Keep current line away from top/bottom borders of the buffer when scrolling
 set scrolloff=5
 " Enable mouse