Smart insertion/replacement algorithm

[salmenf]

Problem: For a given selection S (can be empty/nonempty, TextSelection/NodeSelection/GapSelection) and a fragment F, create a new fragment R that preserves as much of S as possible.

For S -> empty TextSelection:

1. Replace block content in F with equivalent inline content if possible.
2. If F is a single node and can replace the container of S, replace the container of S.
3. Else insert content (and if F contains block content, split container at the block content).

For S -> nonempty TextSelection: ???

For S -> GapSelection:

1. Replace inline content in F with equivalent block content if possible..
2. If F still contains inline content, wrap each inline node in a valid container.
3. Insert F.

For S -> NodeSelection:

1. If selected node fits into F, place it in F.
2. Else if selected node does not fit into F, replace it with F.