Allow merging of content nodes with nodes that can contain content (but are not content themself)

This changeset addresses a problem of pressing backspace in empty list item in order to merge it with list item above it.

Change-Id: I598da6ef24de97b0a7a7bbab6c9ee775aaab460b
Merge "Allow merging of content nodes with nodes that can contain content (but are not content themself)"