Use tmux popup windows for fzf in vim, so that it spans (almost) the
full tmux window. My use-case for this is, that sometimes I open fzf in
a split pane and notice then that the lines are too long for the width
of the pane. Since the fzf window won't resize if I resize the pane
(TODO: issue in fzf.vim), I have to first close it, resize and then
reopen fzf. I hope to fix this by using always the big tmux popup and
eliminating the need to resize at all. The issue remains when vim is in
a split window outside of tmux. I can imagine that this will become
annoying, but we'll see...
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.
