569 support vim sign group and priority (#2786)

* Use sign-group only on supported vim versions.

The sign-group feature is only available in nvim 0.4.0 and vim 8.1.614.

* Add priority to ALE signs.

This allows users to set a priority to ALE signs to take precedence over
other plugin signs.

test/sign/test_linting_sets_signs.vader

@@ -21,7 +21,7 @@ Before:
   let g:ale_set_highlights = 0
   let g:ale_echo_cursor = 0
 
-  sign unplace *
+  call ale#sign#Clear()
 
   function! TestCallback(buffer, output)
     return [
@@ -32,16 +32,20 @@ Before:
 
   function! CollectSigns()
     redir => l:output
-       silent exec 'sign place'
+      if has('nvim-0.4.0') || (v:version >= 801 && has('patch614'))
+        silent exec 'sign place group=ale'
+      else
+        silent exec 'sign place'
+      endif
     redir END
 
     let l:actual_sign_list = []
 
     for l:line in split(l:output, "\n")
-      let l:match = matchlist(l:line, '\v^.*\=(\d+).*\=\d+.*\=(ALE[a-zA-Z]+Sign)')
+      let l:match = matchlist(l:line, ale#sign#ParsePattern())
 
       if len(l:match) > 0
-        call add(l:actual_sign_list, [l:match[1], l:match[2]])
+        call add(l:actual_sign_list, [l:match[1], l:match[3]])
       endif
     endfor
 
@@ -60,7 +64,7 @@ After:
   delfunction CollectSigns
 
   unlet! g:ale_run_synchronously_callbacks
-  sign unplace *
+  call ale#sign#Clear()
   call ale#linter#Reset()
 
 Execute(The signs should be updated after linting is done):