Placeholder insert plugin for RichTextEditor

[lorenzulrich]

In the (Fluid-based) Neos Form Builder, an "Insert placeholder" plugin is bundled that allows to insert placeholders for all the fields of the current form to the finisher text. See https://github.com/neos/form-builder/pull/46.

With support from @mhsdesign, we adapted this for PaperTiger:

The UI code is quite old and it could be solved more elegantly, but I wanted to ask if the maintainers of this package would be interested to include a clean version of this to PaperTiger?

[lorenzulrich]

(We also included an {allFormValues} placeholder to the e-mail finisher that outputs all form values as a definitoin list. This would also be a nice addition, but has no connection to this issue.)

[mhsdesign]

FYI For another client i also build a similar placeholder thing, but with actual ckeditor placeholders, which feel waaay better :D

https://ckeditor.com/docs/ckeditor5/latest/examples/framework/content-placeholder.html

[mficzel]

@lorenzulrich i am totally interested.