Enable Indent in all builds and BlockIndent in the decoupled-document

[Reinmar]

The case with Indent is – we want the indent/outdent buttons in all builds toolbars because it's the only way to indent/outdent lists on mobiles. Related: #868.

The problem – lack of space in their toolbars. Related: #894.

The case with BlockIndent in decoupled-document is quite obvious, I think – it's a document feature.

However, it's an even bigger problem with too many buttons in the toolbar there.

[Reinmar]

Waiting for ckeditor/ckeditor5-ui#451, for now. Though, the decision still has to be made.

Also, perhaps we could have a split button for indentation? If 90% of the time you're indenting things, then perhaps it will be better? It will take less space.

cc @oleq @dkonopka

[dkonopka]

It's true, we should think all the time about space inside the toolbar, but I have one doubt about this new split button: the icon. How we could visualise indent&outdent together or I just misunderstand you and we will make it same as text alignment?

Anyway, some maths here: we could save around 20 pixels (two buttons ~ 60px, when one split button ~40px), so IMHO it's worth to think about it. OTOH, it's still one click more for users to use this feature.

Note: I've researched the most popular editors and there is no usage of indent/outdent action inside a dropdown.

[oleq]

I created a PoC but I don't think we can deliver it in i27:

• It's a considerable piece of code (missing tests),
• There's also the question of whether the main button should be dynamic based on the last command used (UX to discuss).