Use different group names for signs and virtual text (#4704)

Since Neovim commit c4afb9788c4f139eb2e3b7aa4d6a6a20b67ba156, the sign
API uses extmarks internally. Virtual text is already rendered using
extmarks. ALE uses the same group name for both signs and virtual text
and as a result, both are placed in the same extmark group. Since ALE
deletes all extmarks in the virtual text group after all signs have been
placed, no signs are ever shown. This commit fixes this by renaming the
sign group from `ale` to `ale_signs`.

test/sign/test_linting_sets_signs.vader

@@ -37,7 +37,7 @@ Before:
   function! CollectSigns()
     redir => l:output
       if has('nvim-0.4.2') || has('patch-8.1.614')
-        silent exec 'sign place group=ale'
+        silent exec 'sign place group=ale_signs'
       else
         silent exec 'sign place'
       endif