Rename fetch => find for clarity

`fetch` is still defined for backwards compatibility.
Implement ContextNodeModel#clear for unsetting non-ID data

The BranchNodeModel implementation takes an optional argument
`reset` which will remove the child nodes. By default it will
recursively clear the child nodes.
Remove top-level enable/disable checkbox

This caused confusion in the current implementation since it
disabled the root node which made all other nodes inactive even
if nodes were added or changed.