Lexical: Fixed strange paragraph formatting behaviour

Formatting was not persisted on empty paragraphs, and was instead based
upon last format encountered in selection.
This was due to overly-hasty removal of other formatting code, which
this got caught it.
Restored required parts from prior codebase.

Also updated inline format button active indicator to reflect formats
using the above, so correct buttons are shown as active even when just
in an empty paragraph.