Introduce ParagraphButtonUI

[szymonkups]

ParagraphButtonUI should be created, similar to: https://github.com/ckeditor/ckeditor5-heading/issues/104, with the default icon for the paragraph. Plugin should be optional, not required by Paragaph plugin.

[ma2ciek]

We had a F2F talk about providing translations for the plugin. Both ckeditor5-heading and ckeditor5-paragraph packages should use paragraph, so the paragraph msgid should be moved to the ckeditor5-core. (ckeditor5-core is the core package for translations and should be used to prevent duplications). But removing the paragraph msgid from ckeditor5-heading will cause removing all translations for paragraph.

This case has already popped out and its ticket is waiting for the PR: https://github.com/ckeditor/ckeditor5-dev/issues/376.

[Reinmar]

Can we use the translation available in the heading package for now? Or is the scope limited to each package + core?

[ma2ciek]

We check for duplications while we're gathering sources to upload: https://github.com/ckeditor/ckeditor5-dev/blob/6c69057249c8db63f0342fce7643b8707d3dc634/packages/ckeditor5-dev-env/tests/translations/collect-utils.js#L227-L239

[Reinmar]

But it's about repeated context. And is the paragraph feature able to use the translation from the heading feature? Or does that fall into the "repeated context" check?

[Reinmar]

BTW, I may avoid using translations:collect for a while, until we figure out ckeditor/ckeditor5-dev#376. So, for now, it's just a question of whether it's going to build :D

[ma2ciek]

It should build, translations are stored in the key -> translation object for now.