Allow off-canvas links to be rendered by either "default" or "admin" renderer

I'm not really sure that this is a good idea. But, that aside...

1. +++ b/core/modules/outside_in/outside_in.module
   @@ -38,6 +38,7 @@ function outside_in_contextual_links_view_alter(&$element, $items) {
   +      'data-dialog-styles-library' => 'outside_in/drupal.outside_in_styles',
   
   +++ b/core/modules/outside_in/src/Render/MainContent/OffCanvasRender.php
   @@ -52,12 +58,30 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
   +    if (isset($options['drupalStylesLibrary'])) {
   

   It's weird you specify it with hyphens and then it's lowerCamelCase later

2. +++ b/core/modules/outside_in/src/Render/MainContent/OffCanvasRender.php
   @@ -52,12 +58,30 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
   +      if (!isset($options['dialogClass'])) {
   +        $options['dialogClass'] = '';
   +      }
   +      $options['dialogClass'] .= ' ' . Html::cleanCssIdentifier($options['drupalStylesLibrary']);
   

   PHP--

3. +++ b/core/modules/outside_in/tests/modules/offcanvas_test/offcanvas_test.libraries.yml
   @@ -0,0 +1,5 @@
   +      css/paint-it-black.css: {}
   

   lol

4. +++ b/core/modules/outside_in/tests/modules/offcanvas_test/src/Controller/TestController.php
   @@ -92,6 +106,22 @@ public function linksDisplay() {
   +          'data-dialog-styles-library' => 'offcanvas_test/drupal.off_canvas_test_styles',
   

   IMO should be an array of libraries

5. +++ b/core/modules/outside_in/tests/src/FunctionalJavascript/OffCanvasTest.php
   @@ -89,10 +89,20 @@ public function testNarrowWidth() {
          $web_assert->elementAttributeContains('css', '.dialog-offcanvas__main-canvas', 'style', 'padding-right');
    
   +
   +      $this->drupalGet('/offcanvas-test-links');
   

   Extra blank line

+++ b/core/modules/outside_in/src/Tests/Ajax/OffCanvasDialogTest.php
@@ -50,10 +46,26 @@ public function testDialog() {
+      // Emulate going to the JS version of the page and check the JSON response.

Line exceeding 80 characters

+++ b/core/modules/outside_in/src/Render/MainContent/OffCanvasRender.php
@@ -47,15 +57,23 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
+    if ($this->mode == 'admin') {

After talking with @webchick, @Gábor Hojtsy and @yoroy, this seems not necessary for an MVP.

If you want to override the styles globally used in settings tray, you can override or extend the library in your theme/module.

Additionally, if you want to change it each time the tray is opened you can specify dialogClass in the dialogOptions attribute, and have your CSS keyed from that.

I can see the argument that this is not MVP, however it is a valuable thing to have and we should look at putting it in next release.

This is indeed adding a lot of complexity while it's not clear it's actually really solving the problem.

With #2826722: Add a 'fence' around settings tray with aggressive CSS reset., we should have all the protection we need. If that's not enough, then "default" and "admin" also won't do, because they're both theme-specific, which means we'll need one of these "renderers" per theme. AFAIK the whole point of #2826722: Add a 'fence' around settings tray with aggressive CSS reset. is that it's a theme-agnostic reset.

Restoring state from #20.