Implement paragraphs conversion

This patch fixes a problem if a default language is not the first one in the list when doing conversions.

We noticed this in a client project and @Berdir fixed it so credits to him. :)

I have tried to create content + text paragraphs in one language and then switched it to the other language but I can't reproduce it...

I thought that we are missing the alter hook so contrib/custom modules can alter converted paragraphs. It was just undocumented in paragraphs.api.php. Fixed that and while here also fixed the code style errors I noticed.

ParagraphsDemoTest changes are not really related to this. Reverted that.

Providing a fix for the case when a conversion plugin doesn't return paragraph values but a paragraph entity.

This patch fixes an issue with the conversion hook that was not executed on each converted paragraphs.

I updated hook_paragraphs_conversion() to document that the hook is not fired for each translation.

I am providing a patch that improves Drupal\paragraphs\ParagraphsConversionBase::supports() to it checks a user has create access to at least one target paragraph type.

+++ b/src/ParagraphsConversionManager.php
@@ -0,0 +1,185 @@
+          // Loop over the target types and check that the user has create
+          // access to all of them.
+          foreach ($definition['target_types'] as $target_type) {
+            if ($access_control_handler->createAccess($target_type)) {
+              // In case there are allowed types check the target type.
+              if (is_array($allowed_types) && !isset($allowed_types[$target_type])) {
+                $applicable = FALSE;
+              }
+            }
+          }

Just noticed we already have this logic in two places in Drupal\paragraphs\ParagraphsConversionManager. It has a bug as it doesn't set the flag to FALSE if a user has no create access.

In #15, I moved the check into the base class so plugin implementations could decide whether they want to rely on access checks (best practice) or if they're doing something special. If we are enforcing the access check based on the plugin definition we should then move the similar check for the source_type in Drupal\paragraphs\ParagraphsConversionManager IMHO.

Here I fixed the original problem described in #16.1.

Small fix for the error described here #3082542: PHP notice when converting library item

Reroll.

Another reroll.

I propose we add the small button class like we did for other buttons (for Claro) in https://www.drupal.org/node/3111075.

Patch #21 wasn't working for us against 1.13 release.
Attached is one that applies to 1.13. No changes

Thanks for the reroll. FWIW, I wasn't sure about committing this without getting some inputs about it from others. So if it's useful, let me know :)

We've been using this for years. Committed and created a basic documentation page: https://www.drupal.org/docs/contributed-modules/paragraphs/converting-pa...

@Berdir

on documentation page (also in related test),

function name `submitConversion()` should be `convert()`?