Sign and highlights are not set with 'other source' feature

## Information

**VIM version**

```
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Apr 1 2021 21:01:15)
macOS version - x86_64
```

Operating System: macOS 10.15

## What went wrong

I'm making a bridge plugin between [vim-lsp](https://github.com/prabirshrestha/vim-lsp) and ALE. I noticed that sign is not set when calling `ale#other_source#ShowResults`. Sign is not put even if `signcolumn=yes` is set (default value is `auto`).

## Reproducing the bug

Assuming that ALE is installed in `~/.vim/bundle` directory.

1. `echo "a\nb\n" > test.txt
2. `vim -u test_vimrc test.txt`
3. Check sign is put at line 1

```vim
" test_vimrc

set nocompatible
filetype plugin indent on
set rtp+=~/.vim/bundle/ale
let g:ale_set_signs = 1
let g:ale_linters = {}
let g:ale_linters['text'] = ['my-test-linter']
augroup SomeGroupName
autocmd!
autocmd User ALEWantResults call Hook(g:ale_want_results_buffer)
augroup END

function! Hook(buffer) abort
call ale#other_source#StartChecking(a:buffer, 'my-test-linter')
let item = {
\ 'bufnr': a:buffer,
\ 'lnum': 1,
\ 'col': 1,
\ 'vcol': 0,
\ 'text': 'this is dummy error message for buffer ' . a:buffer,
\ 'type': 'E',
\ }
call ale#other_source#ShowResults(a:buffer, 'my-test-linter', [item])
endfunction
```

### :ALEInfo

```

Current Filetype: text
Available Linters: ['alex', 'languagetool', 'proselint', 'redpen', 'textlint', 'vale', 'writegood']
Linter Aliases:
'writegood' -> ['write-good']
Enabled Linters: []
Ignored Linters: []
Suggested Fixers:
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'textlint' - Fix text files with textlint --fix
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
Linter Variables:

Global Variables:

let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = 0
let g:ale_command_wrapper = v:null
let g:ale_completion_delay = v:null
let g:ale_completion_enabled = 0
let g:ale_completion_max_suggestions = v:null
let g:ale_disable_lsp = 0
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 0
let g:ale_fixers = {}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'normal'
let g:ale_linter_aliases = {}
let g:ale_linters = {'text': ['my-test-linter']}
let g:ale_linters_explicit = 0
let g:ale_linters_ignore = {}
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_max_buffer_history_size = v:null
let g:ale_max_signs = -1
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_root = {}
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 0
let g:ale_sign_error = '>>'
let g:ale_sign_info = '--'
let g:ale_sign_offset = 1000000
let g:ale_sign_style_error = '>>'
let g:ale_sign_style_warning = '--'
let g:ale_sign_warning = '--'
let g:ale_sign_highlight_linenrs = 0
let g:ale_statusline_format = v:null
let g:ale_type_map = {}
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 0
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:
```