All buttons should focus the editor before executing their commands

[Reinmar]

Otherwise, the command will be applied, but it may look odd or be invisible to the user because there won't be a blinking caret. For example, bold may be applied on a collapsed selection and that won't have any effect in fact.

Note: this will be consistent wth CKE4, but with that difference that now it's not the command's role to focus the editor. The UI (button) cares about its action which consists of two things – UI (focus) and document action (command).

[oleq]

Most likely https://github.com/ckeditor/ckeditor5-ui/issues/138 will need this to work properly.