Improve behavior when using insertContent

[philippkuehn]

With this PR we try to improve the behavior of insertContent in some cases.

1. When content is inserted into an empty text block, we try to replace the text block instead of inserting the content after the text block. This prevents empty paragraphs for example.
2. After inserting, the cursor is placed at the end of the inserted content instead of after it (fix Incorrect insert of custom node before same custom node #1673)
3. Prevent multiple nodes after inserting a horizontal rule to an empty document (see HorizontalRule extension: inconsistent behavior #1964)
4. Fix error when inserting horizontal rule at the end of the document (fix horizontal-rule extension throws "RangeError: Position x out of range" #2117)