vim: Fix broken highlight of trailing whitespaces

Apply the following:

1. Switch to `matchadd` as `match` can only be used for **one**
   highlight
2. Use an autocommand group for better behaviour when reloading the
   vimrc.
3. Use just one pattern as it seems to work always. (Apparently `\%#` is
   not matched in normal mode)

.config/vim/vimrc.d/50-aesthetics.vim

@@ -40,3 +40,13 @@ if (get(g:, 'loaded_fzf'))
 	" Use a theme for bat in the preview that somewhat resembles onedark
 	let $BAT_THEME='TwoDark'
 endif
+
+" Highlight trailing whitespaces
+" Pattern taken from https://vim.fandom.com/wiki/Highlight_unwanted_spaces
+highlight TrailingWhitespace ctermbg=red guibg=red
+augroup HighlightTrailingWhitespace
+	au!
+	" NOTE: VimEnter is necessary as well as WinNew is not triggered for the
+	" first window created after startup (see :help WinNew)
+	au VimEnter,WinNew * call matchadd("TrailingWhitespace", '\s\+\%#\@<!$')
+augroup END