Auto-fix block-less paragraphs

[Frederico Caldeira Knabben]

When moving the caret inside the editor (on selectionChange), the editor could automatically fix paragraphs with no blocks, resulting in a more accurate WYSIWYG experience, as after all the paragraphs will be fixed on output.

[Garry Yao]

The following steps could be used to test the functionality.

Procedures

1. Open the replace by class example page;
2. Clear all document content by switch to 'source' mode, delete all source codes and switch back;
3. Type some random text, then click on the 'Bold' to apply style;
   • Expected Result: Elements path is shown as : body -> p -> strong.

[Garry Yao]

Limit to fix contents within 'body' only.

[Frederico Caldeira Knabben]

We already have a function for it: range::fixBlock. You should try using it.

[Frederico Caldeira Knabben]

After talking with Garry, we've found out that the patch is doing more than needed, and can be simplified.

[Garry Yao]

I've identified some other problem with the patch though:
Sometimes it's troublesome to have a range start within a bookmark node, e.g. extractContent/insertNode will duplicate the bookmark node which will result in weird result, the patch is proposing a fix to the this case by the 'optimizeBookmark' function.

[Garry Yao]

Including the related latest patch from #3180 for easy reviewing.

[Garry Yao]

Fixing several issues encountered during peer reviewing with Fred.

[Garry Yao]

Fixed with [3549].